The answer is: happy and friendly.
The denotative meaning of a word is the literal sense of the word, exactly as you would find it, for example, on a dictionary.
The denotation of the word jovial is, indeed, happy and friendly. In the sentence above, the teaching style of the teacher is said to be joyful and welcoming, that is to say, jovial, so that the new students immediately felt at ease in this classroom and even felt comfortable enough to participate in discussion.