Other than driving under the Community Bus Permit Scheme, in simple terms (if you regard this as simple!).
If you passed your test before 1st January 1997 you already hold the Category D1 entitlement but with the 101 restriction of 'not for hire or reward'!
If you want the 101 restriction removed so that you can earn money driving a minibus then you will have to take a Category D1 driving test.
Hire or reward encompasses any payment in cash or kind (or on behalf of) passengers which gives them a right to be carried.
If you passed your test after 1st January 1997 you do not have the Category D1 entitlement on your licence and you cannot drive a minibus.
However, you may drive if:
a) you drive for a non-commercial body but not for hire or reward
b) you are over 21
c) have held a car licence for 2 years
d) are providing your service on a voluntary basis
e) the minibus is under 3.5 tonnes
Are you a teacher?
If you are a teacher in the private sector at a non-charitable school, you will need a full 'D1' minibus licence.
If you are a teacher in the state sector and drive under the CBPS, the area can be very complex and grey.
If you drive during school hours it can be considered as for payment. This needs a full 'D1' licence.
If you drive out of school hours it may not, however you are still teaching/supervising/have care and control therefore acting as a teacher you may fall back to the previous 'during school hours'. Claiming the concession will certainly bring you to the notice of the authorities, especially if an accident arose causing damage or injury with the obvious risk of prosecution.
Also please check with your insurance company regarding the use of non - full 'D1' licence holders to see if cover will still be in force.
It's your responsibility to ensure the safety of your children that you transport, and to ensure that you comply with your legal requirements.