How shall i prepare myself for Cleveland Public High School?

I've grown up in private school, now i am going to be attending to a Cleveland public high school. With horrendous ratings. My family has hit a financial roadblock. And i want to graduate, but my Grandpa is telling me to do online school until i'm legally allowed to drop out and get my GED. How will i deal with the transition, and also what should i do over the summer to prepare myself? Has this ever happened to anyone else? Are Cleveland schools really that bad. I just want to get good grades, with no distractions.