* おはようございます (Ohayou gozaimasu): This is the standard greeting for "Good morning." It's perfectly polite and acceptable to use with students, but it doesn't specifically address them as students.
* おはようございます、皆さん (Ohayou gozaimasu, minasan): This means "Good morning, everyone" and is suitable for addressing a class.
* おはようございます、生徒の皆さん (Ohayou gozaimasu, seitono minasan): This translates to "Good morning, everyone (students)." It's more formal and explicitly addresses them as students. However, it might sound a bit stiff depending on the setting.
For most situations, simply using おはようございます (Ohayou gozaimasu) is perfectly appropriate and polite. Only use the more specific options if you need to emphasize that you're speaking to students.