You follow the expected structure of TOEFL essays. Your introdutory para is great in which you give a brief introdution to the topic and specifically state in which side of the argument you are with. That is what they expect.
Then in the body paras you need to give reasons (one reason for each body para) and then support them with specific examples. In your first body para, you provide the following reason but do not support with a specific example;
Students who apply to multiple schools are guaranteed to be accepted, because they are not limited to being rejected by their first choice schools.
your example;
If the student is not accepted by their first choice schools, there is still a good chance that their secondary choice schools will have accepted their application.
this sounds pretty general and not very specific. Provide a specific case... for example;
One of my friends applied to colleges of his first choice only and he was rejected by all of them. He got very disappointed and became very helpless. Had he applied for secondary choice schools, he had a greater probability of being accepted by some college.Second body para is really good and you give specific examples as to why you say students have more options by applying to secondary schools.
Also your conclusion is good.
Overall, you've done a good job. If this is your first essay, you dont have to worry about TOEFL writing task at all. :)