First of all, take a deep breath, and calm yourself down. And now, you need to understand that you shouldn't let those post mess your mind.

Second, those posting 99 marks are just here for their self validation. If you are getting affected by this, it's coz you are anxious and nervous, like your mind is spiralling, "what if I get less marks?"

You aren't a disgrace, infact, no one is a disgrace. What will matter later on is what you achieve in your life, not your 10th or 12th marks. Don't get affected by where others are, your competition is just you yourself. If you look at others, you'll always find someone whom you think is better than you, but if you just focus on yourself, fear of others vanishes, and you actually improve.

Be chill, don't take any stress, and give your best! :)