Please, explain to me how should I behave with a boy who was my best friend and, now, groundless, ignores me?
Do not impose yourself on him, so that you may not be even more ignored by him and then by other people, too. A respectful and proper behavior honor a girl, as it is written in Scripture, even if that text refers primarily to clothes.
Do not hate this guy. In the heart of the Christian must not be hate for anyone. Therefore, live in peace with this guy and if he is your schoolmate or classmate behave with him just as you treat all your colleagues, that is modestly and discreetly.
Do not repay anyone evil for evil. Be careful to do what is right in the eyes of everyone. If it is possible, as far as it depends on you, live at peace with everyone. (Romans 12:17-18)
If that boy is not yet a child of God begin to pray for the salvation of his soul and pray this way for all of your colleagues. So God teaches us in Scripture:
I urge, then, first of all, that petitions, prayers, intercession and thanksgiving be made for all people; (1 Timothy 2:1)
Be wise and protect yourself from such frustrations in the future and never accept to date any guy before marriage. If you find it strange what I tell you, I invite you to read the articles “Dating and successful marriage” Part I and Part II.
Focus on building your relationship with God and learn to live in full obedience to Him and be sure that He will fulfill the promise He made in His word when He said:
The prospect of the righteous is joy, but the hopes of the wicked come to nothing. (Proverbs 10:28)
May God help you live a righteous life!