A group of attentive students are paying attention to a college lecture.
Some students are attending a class.
A group of students must consist of some students; college lectures almost always take place in the context of a class, so while being attentive and paying attention are not strictly synonymous with attending (being present at), it's safe to assume they are in class.
Group of attentive students are some students, and lecture is a form of class.
The students are attending a college lecture class.