In the example, does ‘they’ indicate ‘lessons’?

Perhaps it was because he was now so busy, what with Quidditch practice three evenings a week on top of all his homework, but Harry could hardly believe it when he realized that he'd already been at Hogwarts two months. The castle felt more like home than Privet Drive ever had. His lessons, too, were becoming more and more interesting now that they had mastered the basics.
Hi eipjoo
"They" refers to the students in the class.
Best wishes
Hi..Thank you very much.