To answer this, you would need more information about the students. For example, you could say:
* Son muy trabajadores. (They are very hard-working.)
* Son muy inteligentes. (They are very intelligent.)
* Son muy amables. (They are very friendly.)
* Son muy ruidosos. (They are very noisy.)
* Son muy callados. (They are very quiet.)
* Son muy creativos. (They are very creative.)
You could also describe their specific characteristics:
* A la mayoría les gusta hablar mucho. (Most of them like to talk a lot.)
* Muchos de ellos son muy tímidos. (Many of them are very shy.)
* Son muy motivados para aprender español. (They are very motivated to learn Spanish.)
The answer depends on the specific class and students you are referring to.