school single-lined paper,

Do you mean ordering from your school? coz I can order those single-lined paper in my school. If so, you'll have to ask your stationery mominor/ monitresss.
If you mean buying it from the stationary shop, 商务,大众 also have single-lined paper for sale! how many sheet per bag? they vary with the prize.
But these single-lined paper are mostly about $ 15-$20.
If you're buying the ones with holes, from japan, they'll be about $36 or even more expensive.

同你校務處聯絡，查詢是經他們買到，還是經學生會買"school single paper"。

Do you mean you want to order or tailor make A4 single lined paper for your student-union? or order answer paper for you school?
Anyhow, you need to contact some printers or paper stationery makers who wholesale their products to the book shops and stationery company, so that you can get the best prices.
You can check your record in past for the prices of supplier, or not, please be advised to contact following reliable maker who is running over 30 years in this field:
WINCO GIFTS AND ARTS COLLECTIONS
RM.1563-1575, 15/F., HOI TAI FACTORY ESTATE, KIN FAT STREET, TUEN MUN, HONG KONG.
TEL; 852-24542333. FAX; 852-24548666
EMAIL; [email protected]
You can check their products and qualities at book and stationery shops in HONG KONG everywhere including COMMERCIAL PRESS, POPULAR etc, or co-operation shops of student-union in HKU,HKCU, HKUST ETC under the BRAND name of SUSUKI, WINCO.