What is the ruling on increasing the fee for private lessons in the middle of the academic year and near examinations? Is this considered exploitation or legitimate trade?

Disputes are resolved in a court of law, as it is best equipped to hear all parties, understand the claims and evidence, and issue a judgment, unlike a mufti (religious scholar who issues legal opinions) who hears only one side. A lease contract is valid only if the rent and duration are known. If the lease is فسدت (corrupted or invalidated), then the ajr al-mithl (customary or fair rent) becomes obligatory, which is determined by experts. If the contract is on a monthly basis (mushahara) without a specified duration, it is not binding on either party.