Dim problem! If Lingo says no to that one, just tick 'my answer should be accepted'.
The current "best" translation is "I work in a school.", which matches what you thought:
- Dw i'n gweithio - I work
- mewn - in
- ysgol. - a school.
chi is one word, and chi'n is two: chi + yn.
That yn is the same one that's in the contractions mae e'n ... or dw i'n ....
So basically, you use it before a predicate noun or adjective; or before a main/full verb in the present tense. (But not eisiau/isio, which is a bit special grammatically.)