Bad students: before starting the essay, I think you should clear "bad students" definition. In my view, bad students are always those rebellious, those that are bullies. I also think that those rebellious students are the one having bad academic performance; however, it does not mean that "students with bad academic performance" are bad students.
as they are practiced -> as they are trained in vocational school
Moreover, they save much time -> they can save more time
Whether they go to university, they could not catch up lesson in the class. -> it is likely that when they go to university, they could not catch up with the lessons in class. As a result, the education efficiency of the university is decreased also.
Avoid using "you", it's not usually used in academic writing.
Owing to training -> thanks to the training
Nowadays life is more modern than (than what?) and people use more electrical machine. Therefore, society needs to many worker more. (more workers) Most of people usually choose to learn (come to) university to become the teachers or professor. So, blue worker is lack very many. However, vocational training will do society to have many workers from the unemployed. So, blue-collar workers are seriously lacked in the society. However, if more students attend vocational training, the situation of unemployed workers will be solved.
In general, I think you have presented clear ideas. But it seems that you have troubles in working with coherence and cohesion, vocabulary. Read through my comments and if you have any questions, pls contact me, I can explain in more details