There are never enough discipline on the playground: someone always is getting hurt. Subject verb agreement.

Answer 1

Yes, there is a problem with subject-verb agreement.

In the first clause, the subject is "discipline." What should the verb be?

Answer 2

This sentence above is one terrible example of English. There should be a semicolon after "playground" instead of a colon.

I hope you read a lot of books and newspapers, so that you won't have to learn proper English grammar entirely from books like this one.