IF I could go back in time and give my younger self some advice on how to be a better student, be more successful in school, life, etc, I would definitely tell myself that being involved in everything comes at a cost. It is better to find a few things that you like to do, do them well and often, than feeling stressed because there is so much on your plate at one time. Being a 'Jack of all Trades' it is natural for me to dip my toes in different waters- all at the same time, but that does not mean that I can give 100% to any of them at that time.
While I was able to get good grades (A- average) while in school, I was impressed by how much better I did- and felt about my work- the few times that I scaled back on my activities.
Another piece of advice that I wish that I could bestow upon my younger self would be to learn how to speak up in a group setting when someone is not fulfilling their part of an agreement. Now, this said, the best way to do this would be in a tactful manner- not spreading rumors, belittling, or forming a grudge against someone. There are so many times in life when 'life' gets in the way of a group project and if you can learn how to help each other out in a group setting from an early age, you are bound to be a better co-worker. However, to be truly successful you need to be able to decipher when someone is truly going through something and needs assistance and when he/she is just being lazy- the latter requiring a different skill set to handle the situation.
Other than that, be confident in your abilities and don't be afraid to acknowledge the things that you need help with from others. We all have things that we are good at- we all have things that we are not good at, but if we work to the best of our ability no matter what that is and in what form- we can be the success that we were meant to be.