Children could be hungry if the family doesn't have sufficient money for food. This is the reason for the federal lunch program, which now includes breakfast. For many kids in poverty situations, school lunches are their best meal of the day. Some districts with high poverty numbers try to find ways to feed kids during summer, too.
They could have very few articles of clothing, and the older a kid gets, the more self-conscious he/she is about things like this. If the child is thinking of things like this, he/she is not thinking about schoolwork.
Those things are not minor. Anything like that which interferes with his/her ability to listen, concentrate, pay attention, read, think ... impedes his/her learning.
The whole thing is a spiral, and there are many other factors, including possibly an upsetting home life (one-parent home with that parent maybe working more than one part-time job -- or out of work entirely; any violence at home, whether aimed at the child or not; etc.).
Other teachers may have more specifics to add.
Ms. Sue, if you're not doing good in school, are you suffering from mental stress?
Probably. Doing poorly in school could be part of the cause of mental stress or mental stress could result from doing poorly in school.
chronic stress because it is long lasting compared to short term or acute stress.