* おはようございます (Ohayou gozaimasu): This is the standard way to say "Good morning" in Japanese. It's perfectly polite and appropriate to use with classmates. It doesn't specifically mention them being your classmates, but it's understood in a classroom setting.
* おはようございます、皆さん (Ohayou gozaimasu, minasan): This adds "minasan" (everyone) to the greeting, making it slightly more inclusive. Still polite and suitable for a classroom.
* おはようございます、みんな (Ohayou gozaimasu, minna): This is a more casual version using "minna" (everyone), suitable for close friends in a class. Avoid this with professors or those significantly older than you.
In short, おはようございます (Ohayou gozaimasu) is the safest and most versatile option. Adding 皆さん (minasan) or みんな (minna) is optional, depending on your relationship with the people you're addressing.