I passed the interview and I've done one day's training so far on delivering assemblies. I've not actually been into schools yet with the service.
There are 2 days training. One on assemblies and another on delivering workshops. It's all very scripted which I find reassuring - you really do learn a script for delivering an assembly.
The staff and volunteers I've met so far have been lovely and the training has been good and thorough. You always work in pairs in schools so that you're not on your own and initially you'll always be with someone more experienced. Certainly in my area I think a lot of support is given to volunteers. You may feel a little pushed out of your comfort zone but the NSPCC is very much aware of that and does what they can to assist you and build your confidence.
Hope that helps. Sorry I can't talk about an actual school experience at the moment.