What is the ruling on lying to someone—by impersonating a girl—in order to obtain educational lessons, and what is the expiation for this sin that has affected the life of its perpetrator?

What you did by lying and deceiving is a grave error and a great sin. You have done well by feeling remorse and immediately ceasing this sin. Be sure to firmly resolve not to return to such acts in the future, so that the conditions of repentance are fulfilled. You do not need to ask for forgiveness from the one you lied to; rather, it is sufficient to ask Allah for forgiveness for him and to supplicate for good for him, because informing him might lead to greater harm and a complete break. Have a good opinion of your Lord and remember the vastness of His mercy, for He is Forgiving to those who repent.