To get to Thongsook College from marina house there are a number of options.If there are enough students at marina house at the time of your stay you could share a taxi. This will take about 10-15 minutes and cost, roughly, B100-120. Between 4 people that works out just a little more expensive than the bus and far more efficient.
By bus there are a number of options:
170 - Direct - B16-20 - Every 20-30 minutes - Take 20-30 minutes
This options is quite infrequent so we recommend the following as well
Bus 203 to Pata in Pin Klao (when on board the 203 it will follow Charan Sanit Wong through 1 major junction then turn left at the next major junction, Pata is the next stop)
You need to cross over the road using the footbridge and then take ANY of the following buses to Thongsook College: 40, 123, 124, 127, 149 (with Thanks to William Zuby and Ramsy De Vos for refining these numbers).
You can return on any of the above and use the 203 to complete your journey. Alternatively you can use the 515 which will bring you to Krung Thon Bridge where you can walk back (15 minutes) or catch any of the 18, 110, 170, 175, 203 to complete your journey.