Your pain is real and it is understandable that you feel brokenhearted and alone when lies are being spread about you. To be young and feel that everyone is against you, that you have become a joke and that no one will believe you, is a heavy burden. God sees it, and He hears your prayer.
Scripture warns again and again about deceivers and false witnesses, because lies have always been one of the great curses among people. From the Old Testament prophets through the teachings of Jesus and the apostles, we are told to expect false accusations. They are not always easy to spot. A lie is often most convincing when it is mixed with a little truth, and those who spread it do not wear a sign to show what they are doing. That is why false reports can be so destructive. They create a cloud of suspicion that hurts the innocent and makes others unsure whom to believe.
You said you know in your heart that what is being said about you is not true, and that you have not done these things. Hold fast to that quiet knowledge and to what God knows. People may be quick to believe a rumor, but God is never deceived. He knows you completely, He knows the truth about what has and has not happened, and He loves you as His daughter. His view of you is not shaped by gossip. When others misjudge, He remains the faithful witness who sees clearly.
Jesus Himself knew what it was to have false things spoken against Him. Others twisted His words and brought false charges against Him. He told His followers that they would be blessed even when people speak falsely against them on His account, and that God would be their defender. The Psalms are full of prayers from those who were slandered and harassed, asking God to be a refuge, to silence lying lips, and to let truth come to light.
I pray with you that the Lord will protect you from this harassment and bring it to an end. May He place a guard over your reputation and your heart, comfort you where you have been hurt, and surround you with trustworthy friends and adults who will listen and believe the truth. May He also work in the hearts of those who are spreading these rumors, bringing conviction where there is wrongdoing and leading them to turn from putting you down to seeking what is good and true. And where there are people who need to be removed from close influence in your life for your safety and peace, I pray He will graciously do that.
In the days ahead, keep bringing your heart honestly to the Lord in prayer, and do not carry this alone. Talk with a trusted parent, family member, counselor, or pastor who can walk with you, offer wisdom, and help protect you at school and among peers. Do not try to repay lies with lies, but continue to be yourself, to live quietly and truthfully before God. He is your defender, and in time He brings truth into the light.
May the Lord give you peace that is stronger than rumors, remind you daily that your worth is found in being known and loved by Him through Jesus Christ, and give you courage to keep doing what is right even while others speak wrongly.