what could be a good explanation for the phrase; "catch the student being good"?

It is easy to catch a person doing the wrong thing or being bad. It is even easier to criticize him. It is much harder to look for the good things a person does; however, positive re-enforcement for even the smallest correct actions will lead to the worst student trying just enough harder that he will finally understand.

Some students (mainly children) behave badly because they want to call attention, they like the attention they receive when they are disruptive (even if is the teacher's disapproval).

So, to "catch'em being good" is to praise them and give them attention when they are not being naughty at classroom, as well as praising other students that are good, so the disruptive student sees that being so is not good, and that they receive attention by behaving well.

It is a very good technique and worked with me!!

=D