Some think it necessary to do so.Firstly,it's more effective to study with a teacher's help than by themselves.Secondly,you can have more chances to communicate with teachers(there are more interaction between students and teachers.) .Besides,you can make up for what you miss or fail to understand in classes on weekdays.Others think it unnecessary.For one thing,students can easily form the habit of dependence.For another,students need time for rest and recreation at weekend.What's worse,attending training classes or hiring tutors even causes some students not to listen attentively in their regular classes.In my opinion,whether a training class or tutor is needed just depends.If you are really very weak at or interested in a certain subject,maybe it's OK for you.But be sure to choose a good and suitable class or teacher,otherwise it would be a waste of time and money.