I am often asking my children to work more quickly.

The opposite of slow is fast. What do you mean by "he is slow"? Does he move slowly? Probably can't fix that. Is he stupid? Probably can't fix that. Does he not study enough so he doesn't know his material well? He needs to study more. Probably can't fix that. Is he shy and doesn't participate? You can ask him to participate more. Probably can't fix that. People generally can't or won't change their attitudes. We say "A leopard can't change his spots."

By the way , what does "too slow in class "means anyway ? It means the student is not following the lessons well enough. Want them to be more active > more participatory. do things faster > alert to the need doing things faster. I want to know the adjectives that can be used describe a student who is" too slow" in class, adjectives that will indicate the student active participation as well as being alert to things that are required to be done. This is because students sometimes need to have qualities in order to learn whatever you are teaching them.

Yes, energetic is the correct word to use. Also, you can say "you need to be more active in the group".