They're both grammatically correct, but it doesn't sound quite natural. If we imagine that you're currently in class, the natural thing to say would be: "You look tired" or "You look tired today". When you add "in class", it sounds a bit off. If we imagine that you already left class, then you could say: "You looked tired in today's class" or "You looked tired in class today". Just to reiterate, your sentences are both technically correct, I just can't think of a context where they would sound natural.

You look(ed) tired in class today - it should be past tense You look tired in today's class

You looked (past tense) tired in class today. (earlier, before now) You look tired in today's class (right now class is happening)