Formal:
* Le professeur est amical. (This is the most literal and formal translation.)
* Le professeur est sympathique. (This is a bit more natural and means the teacher is pleasant and easy to get along with.)
Informal:
* Le prof est sympa. (This is a casual way to say the teacher is friendly, and it's more common among students.)
* Le prof est cool. (This is even more casual and means the teacher is "cool" or "laid-back".)
You can also use adjectives like:
* gentil(le) (kind)
* agréable (pleasant)
* chaleureux(seuse) (warm)
Here are some example sentences:
* Le professeur d'histoire est très amical avec ses élèves. (The history teacher is very friendly with his students.)
* J'aime beaucoup mon professeur de français, elle est très sympathique. (I really like my French teacher, she's very pleasant.)
* Le prof de maths est vraiment cool, on peut lui poser toutes les questions qu'on veut. (The math teacher is really cool, we can ask him any questions we want.)
The best choice will depend on the specific situation and your relationship with the teacher.