Yes, VAF1B is the right form. Completing it online means that you can make an appointment with the VAC to hand in supporting docs etc., which may save a wait if they are busy.
Part 5. Presumably she is a school student? Then tick the 'student' box for 5.1. and fill in her school details in the box. Then answer 5.2 and if the answer is 'none' then, as the form says, go to 5.9. 5.10 - 5.14 is presumably 'none' for each, and the rest is straightforward.
As well as documents showing that she will be financially supported and adequately accommodated in the UK you will also need a letter of consent to the trip from her parent(s) or legal guardian. A letter from her school confirming her attendance and when she is due back will also help. Plus the other documents on the checklist.
See VAT3 Special Visitor: Child visitors for more details.