“I’ll only tell one person” is how everyone ends up knowing.

Arithmetic defeats that promise immediately. Everybody in the chain makes the same reasonable-sounding exception, and one person telling one person telling one person covers a school in roughly two days. Nobody in the sequence thinks of themselves as the leak. They were all only telling one person, which is how the limit becomes a formality.

Treat anything told to you in confidence as having no allowed exceptions, including your closest friend and your sibling. If you genuinely need advice, strip the names out first. Where it has already travelled further than you meant, telling the person yourself beats letting them find out sideways.