Jock, nerd, queen bee are just stereotypes, but do they happen in real life? What about the portrayal of bullying?

Yes, but not as they appear in the movies. Bullying is also overblown.

And are there really popular attractive people that everyone wants to befriend?

Kinda

Do you really go to whatever class you chose? Do you have showers for PE class? And dissect frogs?

Yes (although there are requirements you have to meet for graduation); no because you don't have time to use the showers during class; depends on your school

And have these awesome extracurricular activities of your choice like gymnastics, a cappella, various sports? All for free?

Yes (although costs depends based on school)

Do you really not wear uniforms and have parties in your big suburban houses?

No uniforms, but parties are a thing