DRAMATIC LIGHTING INTENSIFIES
That sidenote of "people might want to use you to get information" was something I attempted to imply in the previous comic's dialogue, but it got a bit too clunky. Granted, it's a conclusion one could reach based on what was said, but I wanted it more blatantly addressed.
I'm fine with plots that involve characters keeping secrets for very good, or simply realistically human, reasons. Sarah trying to keep "I have magic" from her friends, however, would be virtually impossible. Use it around Tedd while he's using his ability? Caught. Justin's new potential boyfriend "looks" at her? Caught.
Heck, as unreliable as Elliot and Ellen's martial arts senses are, there's even a chance they could pick up on something.
All that, and there's always the chance that someone will notice Sarah's mark while she's getting ready for gym and start spreading rumors.
In short: Even if Sarah wanted to hide this from her friends, it would likely backfire very quickly.