I highly recommend SMILETutor Tuition Agency. All their tutors are minimum 'A' grade in their subjects, and their coordinates respond really fast and very friendly. You can check them out at www.findhometuition.com

Copied from their website:

Finding for a reliable private tutor in Singapore? With many options available, it is a challenging task to find the right tutor. But, as you’re here with us, we assure to help you with our highly committed services. We believe that every student deserves a smile - not get bogged down by academic pressure.

SMILETutor strives to exceed the expectations of both parents and students. We constantly revise our system to match you with the best tutor to suit your demands. We help you find home tuition with manually certified registered home tutors in any subject for different age groups: Pre-School, Primary, Secondary, and Junior College.

You can replace tutors at any time and they have a large pool of tutors.. service is very efficient so you dont't have to worry.