Is it illegal for teachers with foreign exchange certificates to collect membership fees?

Generally, it is illegal to evade the private operation of SAFE, because once you evade supervision, it means that your funds will become unaccounted for and be suspected of "money laundering".

Foreign exchange, called foreign currency in English, is a creditor's right held by monetary management organs (central bank, monetary management institution, foreign exchange stabilization fund and Ministry of Finance) in the form of bank deposits, treasury bonds and long-term and short-term government securities. Can be used when the balance of payments is in deficit.