You may want to take 186 from Bouna Vista MRT station instead of walking.
If not, take the Circle Line from Buona Vista MRT to Farrer Road MRT.
From there, you can walk to St Margaret's Secondary.

For the Sec 1 brasses & percussion: 11 June, 9.30am-12.30pm
For the Sec 1 woodwinds: 11 June, 2pm-5pm
For the Sec 2 woodwinds, brasses & percussion: 12 June, 9.30am-12.30pm