The sentence is a grammatical disaster. First, it should be "greatest," since it's singular and definite. "School" is misspelled, and the sentence wrongly states that you are a committee. I'll try to offer some suggestions:
As to the structure, think of it as "What (would you say) has been your great achievement on the school committee?" In written English, I believe it's better style to word it "What would you say your greatest achievement on the school committee has been?"