I do all my giving direct from my salary into a charity account - and can donate from there by electronic means from the account site, or by cheque.
For something like this I intend to give a charity cheque to the school and for them then to order the PIs directly.
Saves a lot of the hassle.
Charity accounts I've used:
Currently: Charities Trust http://www.charitiestrust.org/
In the past: Charities Aid Foundation: https://www.cafonline.org/
Likewise, I have used CAF in the past (I used to work there), then Charity Giving, and now Charities Trust! There are several others. Agreed, this is an easy way to give, but keep an eye on the charges they levy - it does very from scheme to scheme, and not all employers support such schemes.
Note - most but not all schools are classed as charitable and qualify for Gift Aid.
Also, I don't think these schemes work for business users.