Try living on campus .. that would be most convenient.
Anywhere along the KCR route (
http://www.kcrc.com/eng/services/routemap/index.asp) between Tai Wo - Shatin or Ma On Shan will take ~ 20 mins.
Kowloon (TST) to CUHK - 30+ mins by MTR & KCR
HK island - 40-45 mins
Anywhere else - Gold Coast, Tuen Mun, Laguna, etc - 1 hr++
Above times are for public transport. Add another 10+ mins to travel within campus .. depending on where your school is located.
You can also find chartered buses from few areas over HK .. coming to school at 8:30 and leaving at 5/5:30 or so.. but that may be too restrictive.