Earn it first
Reviews follow results and reliability: turn up on time, give parents short progress updates and make the student feel capable.
Ask at the right moment
Ask after a visible success. A short message works: thank them, mention what went well, and ask if they would share a sentence or two about the experience.
Make it easy
Send one link and say what is helpful to mention, such as the subject, the level and what changed. Specific reviews persuade more than generic praise.
Ask for referrals separately
Referrals are a different request. Mention that you have a few spaces and would be glad to help friends or classmates, without pressure.
Keep it honest
Never write reviews yourself or pay for them. Respond politely to any critical feedback and use it to improve.