What's the difference between junior high and high school?

I'm in 8th grade and its almost over! Im so excited..(:. Okay, so my high school is kind of small compared to the other schools and we only have a few junior highs going into my highschool. So how many people on average do you think would be in my classes from junior high? I kinda wanna start over and reinvent myself so are their gonna be a ton of people from junior high in my classes? Also, tell me about classes, and all that kinda important stufff(: