Translation:My mother writes a note because my brother does not go to school.
ar is a preposition. More often than not, English uses the preoposition "on" where Irish uses the preposition ar, but that doesn't mean that ar means "on", it just means that ore often than not, English uses "on" where Irish uses ar. But sometimes English uses a different preposition when Irish uses ar (ar neamh, ar fheabhas, ar oscailt, ar maidin, etc).
I suppose if you consider English's use of "at" to be idiomatic in the phrase "at school", you could call the use of ar in ar scoil idiomatic too, but I'm not sure that that serves any purpose.
From the FGB entry for ar:
.... (Attendance) Ar scoil, ar an aonach, ar an Aifreann, at school, at the fair, at Mass. .... (Activity) Ar an obair seo, at this work. Ar a urnaí, at his prayers. Ar mo dhícheall, doing my best. Ar an ord, working with the sledge-hammer. Ar na scadáin, fishing for herring. Ar a bainne, ar a bleán, (of cow) giving milk, being milked.