I've already given all of the caveats--this is simply my experience and so on, but there is one more important one I need to add. I teach in a K-8 school, so 8th graders are functionally seniors. They have a great deal of responsibility, as well as commensurate opportunities for leadership. This makes a difference, I think, in how they act. I suspect that students in a 7-9 Junior High School model might act differently in some ways. So just keep that in mind.
The good news is that most 8th graders don't experience the vast ups and downs common in 7th grade. Most likely, there will be a degree of stability. The bad news is that childhood is just about over.
This can be good and bad, or rather, good and bittersweet. For example, 8th graders will probably not be quite as irrationally embarrassed of you. However, they will probably not be quite as affectionate or outwardly loving. The good news is that they will probably be more autonomous and independent with things like school work and some of their own maintenance. However, they will also show a commensurate degree of independence and autonomy in emotional ways. They will need you a little less. That can be a painful change. I would not that this may be especially pronounced in boys.
Don't make the mistake of taking this personally. It is part of the natural process of growing up, but it can smart and sting a parent. My suggestion is to grieve privately, don't make your child feel guilty for this natural process.
The way I think of it is this: when my students (or my own children) are 6th graders, I am helping them through their last year of childhood. In 7th grade, I'm helping walk them through a very awkward and painful transition. In 8th grade, I'm now teaching (or parenting) baby adults.
They are very, very young adults, but they are beginning to think in adult terms and live in a more adult world by 8th grade.
Note: You are not done being a parent. Far from it. In fact, the decisions that your 8th grader will make will have a huge impact on the rest of their life. Success in school will now be closely linked to college and a career. Their decisions with regards to friends and social activities have huge implications as drugs, alcohol, and sexual activity began to be real possibilities.
So, this is not the time to check out. They need your guidance more than ever. But there will be a shift in that relationship. You are now laying the groundwork for the relationship you will have for the rest of their lives. You will, in large measure, have to earn their trust and respect now. They will listen to you if you reason with them, but you will be one of many voices, not the center of their world. You will have to do a little more convincing and sales work as opposed to simply laying down the law (although there are times when this is necessary).
They have much more capacity, and as the tilt-a-whirl of adolescence slows down a bit, they will stop being so inwardly focused and might start looking outward. In 7th grade they were consumed with what their peers thought. Their relationships were likely somewhat unstable and tumultuous.
Now, with growing stability, they will begin to make solid friendships again. They may make some new friendships or rekindle old ones, but they will start looking out more.
They will probably start moving from simple imitation of romantic relationships and some crushes to very young love. It's not the real deal, and there is still a lot of immaturity involved, but they will start to have deep feelings for their friends, and that includes some romantic feelings as well.
As I was writing this, I asked some of my 8th graders what they wished their parents knew. One of the things they said was, "Everyone time I talk to a boy, don't assume I'm in love with him." That's good counsel. Not every boy-girl interaction is romantic. I've noticed that many boy-girl friendships have a degree of chemistry and flirtatiousness, but are mostly platonic.
Relationships will be somewhat ambiguous as they will be doing a lot of exploration. A girl might be friends with many boys, and consider them only friends, but still be flirtatious and even mildly affectionate. Don't use black-and-white, either-or terminology to understand your child's relationships at this age because there is a great deal of experimentation and changing going on.
While this can be true of boys as well, my sense is that many boys start being interested in getting a girlfriend, looking for one specific girl. They can have female friends as well, but I have noticed that many of them begin behaviors we would consider dating.
Boys will probably hit their physical growth this year. In fact, it might be the first time some of them are bigger than the girls. I've noticed that the girls are usually the dominant group leaders in younger grades--when influence is exerted through emotional and social means. Things get a bit more equal as the boys get their size.
8th graders will see themselves as being adults. They will not see themselves as being baby adults, but the real deal. They will not understand their limitations or see they still have a long way to go. Consequently, they will chafe at some boundaries and limits.
I have found that it is helpful to give as much latitude and autonomy as I can. This helps establish some credit. Then, on areas where there cannot be latitude or autonomy, I am unyielding. I have also found that explaining why I do things or require things make a big difference. 8th graders will often disagree but go along if they feel you are being fair as opposed to arbitrary.
They see themselves as adults and respond well to responsibility. I find that saying, "If you want x privilege, you need to do abc" they will often work very hard to comply with the requirements. They can sometimes be motivated by appeals to do good work simply for the sake of doing good work, but do better with incentives and feeling challenged.
They respond well to humor (sarcasm, especially), reason, logic, incentives, and anything that appeals to their growing sense of being an adult. They respond poorly to force, compulsion, constraint that seems arbitrary or pointless, and anything that makes them think you see them as children.
One of the other things my 8th graders told me is that they hate being peppered with questions: "Where are you going? Who's going to be there? What did you do? Was it fun? Who said what?" 8th graders will want a degree of privacy. They feel able to direct their own lives and so won't want intrusions into what they see as their sovereign affairs.
But, a parent has to know some of this information, so I don't suggest not asking them anything. I think it's good for them to feel accountable. Remember, they can make adult-level mistakes (in terms of ruining their lives) but don't yet have adult-level life experience or judgment.
I think a good strategy is to say, "Look, I have to know what you are going to do and who's going to be there. As long as you live under my roof, I need to know x, y, and z. If you will tell me what I need to know, I won't bug you about the other details."
They often find it off-putting when adults try to be too fun or cool, or get too involved in who is going out with whom and so on. One of the best things you can do to get along with your child at this age is to be an adult. Don't try to be cool or younger than you are or fun. They start to smell that out and really don't like it. As they mature, they want someone mature to look up to.
Working with 8th graders can be rewarding. Their bodies are mature enough that they can sing well or execute athletics successfully on a whole new level. They are confident enough that they can start to take some risks, and they can start to think at new and more sophisticated levels.
As long as you understand that this comes with a corresponding pull away emotionally, and the fact that they want adult privileges, it can be a really wonderful time.