General Illness/Discomfort:
* sick - Miguel went home from school because he felt sick.
* ill - Miguel went home from school because he felt ill.
* unwell - Miguel went home from school because he felt unwell.
* under the weather - Miguel went home from school because he felt under the weather.
Emotional Distress:
* sad - Miguel went home from school because he felt sad.
* upset - Miguel went home from school because he felt upset.
* down - Miguel went home from school because he felt down.
* miserable - Miguel went home from school because he felt miserable.
Physical Discomfort:
* nauseous - Miguel went home from school because he felt nauseous.
* dizzy - Miguel went home from school because he felt dizzy.
* weak - Miguel went home from school because he felt weak.
* aching - Miguel went home from school because he felt aching.
The best substitute depends on the context and the specific feeling you want to convey.