Just remember the golden rule of unblocked gaming: Close the tab when the teacher walks by. Mute the sound. And always, always do your real work first.
Generally, you are violating your institution's Acceptable Use Policy (AUP), but you are not violating any federal or state law. The worst consequence is a temporary network suspension or a warning from a teacher/boss.
But what exactly is "Unblocked Games Premium 76 Top"? Is it a single website? A collection of the best titles? And, most importantly, is it safe?