table: 1-11318462-29
columns: School, Location, Enrolment, Founded, Denomination, Day/Boarding, School Colours, Abbreviation, In competition since
Q: What is the enrollment of STM which has been in competition since 1990?
A: SELECT COUNT Enrolment FROM 1-11318462-29 WHERE In competition since = 1990 AND Abbreviation = 'STM'