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You can mention different ways students use to stay pretty such as dieting, saving up lunch money to pay for make-up classes or the gym, go for plastic surgery etc.

You can then mention the impact of bad health on students' studies, both long term and short term

short term:
missing schools due to health problems can affect their studies, e.g. poor exam results
feeling hungry during lesson can affect their concentration during lesson;
plastic surgery may go wrong leaving longlasting effect on the student's health;

long term:
lack of nutrition can affect their growth and poor bone structure etc
lead to other illnesses which may affect the rest of their life.
For those who think being tan is pretty, may expose to the sun for long period of time and increase the chance of developing skin cancer.
Some students may think taking up smoking can help them to become slim, then you can mention the adverse effect of smoking.

You just need to find out some common illnesses such as anorexia that can be caused by dieting and lack of nutrition and then elaborate on them. I think there are more arguments on the negative side than the positive side.