Is it illegal for teachers to do sideline work?

Legal analysis: as long as you don't do anything illegal, it's not illegal.

Legal basis: Article 6 of the Measures for Handling Teachers' Violation of Professional Ethics in Primary and Secondary Schools should adhere to the principle of fairness and justice, and combine education with punishment; It should be adapted to the nature, circumstances and degree of harm of its violation of professional ethics; It is necessary to have clear facts, conclusive evidence, accurate nature, proper handling, legal procedures and complete procedures.