So the Police may use water cannons aginst the student protesters ,why not?

If they start using water cannons then things will get much much worse. People have a right to protest and that's what the students did, it wasn't a riot. At any protest you will get a certain element that attend with the sole purpose of causing trouble, but the vast majority of those that attended protested peacefully and shouldn't be bracketed with the others. The police also need to learn from their mistakes, the kettling and riding horses into a crowd that was mainly peaceful and had youngsters amongst it. It must have been evident some some people on the march would try to get into parliament square, so they should have put up proper barricades if they wanted to stop them.