I've been that way. Since grade 7, I would skip a lot of school.
So other than going to school on a regular basis, here are some things you can do to try and get better grades
1) if you know you will be missing school, sya because of an appointment, tell the teacher a few days in advance and ask them to prepare all the homework for you so that you can do everything while you are gone
2) get all your teachers emails. If you are sick one day, get your mom or dad to send them an email to let your teachers know you are sick for the day, and to ask the teachers what you would be doing those classes and for any assignments or if any notes will be taken. Then if there are any handouts, get your parents to pick them up at school.
3) Have a few friends in every class that you know have good attendance and borrow their notes if you miss classes, and get them to explain anything you don't understand if you don't get a chance to ask the teachers.
4) if you can go early in the mornings or stay after school for any help on homework or just to ask any questions, do so.
5) Be sure to complete homework assigned DURING class if time is given instead of socializing. If that's too hard, then do both. And then when you are finished, or almost finished, double check with the teacher to make sure that you understood everything and that it was done properly.
6) on assignments that you are given say.. a week to complete, try and do as much as possible in the first few days, especially over weekends. And try and be as creative as possible. If it were a personal response or something for English, perhaps write up as much as possible over the weekend or during that night, and then ask your teacher to look over your current work to see if they have any pointers on what else you could do to improve or to see if there is anything you should add more of, remove, etc. That way you might have a better chance of offering a better completed assignment
7) If you do poorly on something, ask the teacher if you can possibly redo it or do something similar. My English teacher told my class that if there is ever a written assignment handed in that gets a low mark, we can ask her to let another teacher take a look and give their mark on it, or that we can rewrite it. See if your teachers will let you do this.
8) Be sure to keep a very organized binder with all your old assignments and with all your notes.
9) if you have any good friends in your classes, invite them over a few times a week or over weekends to do a group study time or to help each other out with homework. You can also do this over MSN or Facebook or something.
Basically, keep good verbal communication with all your teachers and with classmates, and be sure to go to your teachers anytime you want/need extra help or you want some advice on homework, or to ask what you can do in order to get a better grade.