"senior school" means: British English term for a secondary school, typically covering ages 11 to 16 or 18 depending on the institution. Senior school follows prim.... This is a fairly neutral word with no inherent risk attached. There is no real cause for parental concern; it is descriptive vocabulary rather than something dangerous. If your child uses it, context will usually make the meaning clear. A brief, curious question about where they heard it is generally enough to know whether to follow up.