Gender dysphoria is a discomfort for people who feel their gender is different from their assigned sex at birth. There are three main types of gender dysphoria: physical, mental and social.
Physical gender dysphoria is when you feel uncomfortable in your body because of your sex assigned at birth. This would not be because you feel like you should be a different weight, or you don't like your eye shape, because those things don't make you your gender.
Social dysphoria is when you are uncomfortable when being called by the pronouns and/or name you were given at birth. So, If you use he/him pronouns and your friend says "this is my friend, she is my neighbor" that might make you uncomfortable.
Mental dysphoria is when you're afraid to embrace the fact that you're trans. A good example of this is being afraid to change your name, pronouns, or physical appearance such as getting a haircut or starting hormones.
Kids all over the world are told that they have to love themselves, that no matter what shape or size, they are beautiful. Gender dysphoria plays into that. When they first get their hair done, put on a binder, or try more masculine or feminine cloths for the first time, something changes. suddenly, they can see they are beautiful, and that the only thing stopping them from seeing that was the way society told them they were ugly. Overall, just like anything else, gender dysphoria is different for everyone.
There are a lot of reasons why a kid wouldn't want to go to school, like they want to play video games and be in bed. This isn't the best way to tell, but if you notice a sudden change in social interaction and more baggy clothing, this might be a sign.
For some, seeing their body can be difficult with gender dysphoria. If they feel uncomfortable taking a shower and ask about things like wearing a swim suit, this might be a sign.
Maybe they feel more comfortable wearing something that the opposite gender might wear because it causes a sense of gender euphoria.
Often, Transgender people are used to hearing invalidating things, this can cause bad thoughts and mental health.