There was no school in Mullaghbrack (according to Griffiths Valuation). Search the contemporaneous maps on Griffiths for the townlands adjacent to Mullaghbrack and you will see where there were schools. There was one in Creenagh for example. That wasn’t too far away.
Also if you search the 1901 census for Leitrim, with the DED of Mohill, and the occupation “teacher” you will find 12 teachers in the area. That also tells you where there were probably schools.
Surviving attendance records may be on Findmypast.