What is the ruling on being shy to speak with girls at school? Is it correct to avoid speaking with them for fear of falling into sin? And what should a student do when a teacher advises him not to miss the opportunity to speak with a girl?

It is obligatory to leave studying in mixed-gender schools unless there is a necessity. If a necessity exists, then one must be careful not to sit next to girls and not to respond to a teacher's words that invite indecency. One must also adhere to the truth, seek righteous companionship, and seek the pleasure of Allah Almighty even if people become displeased. Indeed, modesty is entirely good and brings nothing but good; it is part of faith.