nouns - Is the word 'group' singular or plural in sentence: "My group of tenth-graders is/are so well behaved?"

I have read similar questions on this forum and as per best of my knowledge it should be considered plural because it's referring to every student of class.

I read this sentence in my grammar book: "My group of tenth-graders is so well behaved?"

But my instincts says the word "is" should not be used. I'm confused now. Please confirm whether I'm right or wrong.