Formal:
* Je suis heureux/heureuse d'être présent/présente en classe aujourd'hui. (This is the most formal option, literally meaning "I am happy to be present in class today.")
* Je suis ravi(e) d'être en classe aujourd'hui. (This is slightly less formal than the first option, meaning "I am delighted to be in class today.")
Informal:
* Je suis content(e) d'être là aujourd'hui. (This is a more casual way of saying "I am happy to be here today.")
* Je suis heureux/heureuse d'être en cours aujourd'hui. (This is a more informal way of saying "I am happy to be in class today.")
More natural and idiomatic:
* C'est bien d'être en classe aujourd'hui. (This means "It's good to be in class today" and is a natural and informal way to express happiness.)
You can choose the option that best fits the situation and your relationship with the people you are speaking to.