how do you gain weight? HELPP

I just did my physical exam today as it is a requirement for the enrollment. I've been receiving harsh jokes about my body, bullying in simple terms but in a humorous way, among my peers saying that I am too skinny. Just so you know, I'm a big eater but I don't gain any weight. I did my research about this before and fast metabolism is the reason. Any suggestion guys? This is affecting my mental health.