I want to attend a online school over the summer to boost my GPA what are some of the best providers?

It'd be best to try your local community college, they offer dual credit and it would grant you both high school and college credit. Depending on your school, it may be weighted differently (higher). You might also want to heck if your district offers summer school, as most do, it'll cut down on the hassle of trying to get the credit transferred to your school.