School with a dull sound (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

SCH (abbreviation for school) + W (abbreviation for with) + A . A simple vowel sound, like “uh” or the unstressed second syllable of “comma”: dull in the sense that it’s an indistinct sound produced by a relaxed mouth, without any of the physical adjustments required to produce a more specific vowel.