Is it necessary to change schools after repenting from speaking with girls there?

You have done well by repenting from unlawful communication. You must appreciate this blessing and thank Allah for it by adhering to righteousness, seeking beneficial knowledge, and performing good deeds. Studying in mixed-gender institutions is not permissible except out of necessity or need, provided there is security from temptation, and one observes modesty, lowers the gaze, guards their chastity, minimizes evil, and chooses righteous company.