Your initial conviction was right. I'm very sorry that you gave into that temptation. God is merciful to forgive, and it is a great thing that you've recognised that it was wrong and are determined to live for Christ now. That is very precious in the Lord's eyes. From experience I've learned that God quite often seems to orchestrate negative consequences for our sins, to teach us. "The Lord disciplines the one he loves, and he chastens everyone he accepts as his son". You were chastened by the fact that word got round and the consequences were negative to you. Do you realise how fickle people's opinions are? You would have been damned if you did grind or damned if you didn't, and that voice of temptation hid the former to persuade you by the latter to side against the Lord. I think your chastisement is a sign that the Lord loves you as his son. Keep walking in him. You have my respect.
I think it doesn't matter whether you're in highschool or not; sexual purity is a matter of spiritual obedience, and I'd have thought that it applies to anyone above the age of puberty who cares about God's glory and their soul: sin isn't less sinful or damaging because it's committed by someone who is less mature in years. Timothy was young, and yet the Apostle Paul sent him out to be a church leader, and told him to let nobody think less of him because of his youth: being young had nothing to do with his responsibility to guide that church in holiness.
Moreover, as a matter of practical wisdom I've learned through life that what you do shapes the trajectory of what you go on to do. And habits make the man. I believe that a fornicator is not one who fornicates once and repents and is forgiven and washed clean through the covenant of Christ's blood, with a view to never defiling himself again; a fornicator is one who makes a practice of fornicating.
You did right to repent, and the Lord has chastised you in love. Take it in your stride, and don't listen to the peer pressure any more. If it's better to lose you right hand than to be sent wholly into hell through that besetting sin that it keeps causing you to commit to your estrangement from God, then you can afford to lose their esteem of you. As I said above, it's fickle anyway.
What God has forgiven, it is wrong of them to keep judging you for, if that's what they're doing. Stand strong in the Lord. Satan is called both the Tempter and the Accuser, and peer persuasion is very much one of his instruments. Don't listen to them. You are forgiven. Walk in it.
