Video games actually helped me a lot. It is a nice simple distraction and depending on the game, I got a small sense of achievement from it. One of my all time favorites is the portal series. A documentary on netflix that challenged how I think was very nice in helping me use my brain without having to worry about getting charged emotions. HN is the most useful social media sites I use and helps me stay in touch with the current state of technology.
Making screen time a boogeyman is not a useful way to help yourself address underlying issues. People escape using screen time, and in moderation it's healthy. Taking a break isn't bad. Mental health issue cause a drive to escape using technology. It's not the other way around.
Most people in school are young enough where deficiencies aren't a major issue unless they are also restricting their diet due to finances or other issues. Restricting can cause malnutrition but it's far from common. Just having an, on average, well balanced diet is more than enough. If there's a reason to suspect an underlying health condition, talk with a doctor.
I agree that cardio is amazing but other from of sports and activity is great too. Resistance training and yoga are exceptional as well. Archery is one of my favorites. The important thing is time and consistency, so finding and doing a physical activity you like is all that matters.
Getting checked out with a mental healthcare specialist pre-emptively is important. People have a yearly physical, I'd argue a yearly mental health check should be strongly encouraged too.