From a parent's perspective, most will automatically jump to conclusion that you're playing a game if you're using the computer. It's in their nature to do that because of what they know about our generation as gamers and what not.
From personal experience, as long as you show the results of doing well in school, your parents will eventually not nag you so much about being on the computer for so long. I also went out and did a lot of volunteer work before so it's a sign that I'm not addicted to the computer.
Of course, you should know the limits of staying on a computer. The longer your eyes are exposed to something like a computer, your eyes may (or maybe not) get worse to a certain extent. It's always good to stop using the computer and let your eyes relax.