XingMing it's very kind of you to volunteer your services and I'm sure that you meant very well, but it's not as simple as schools asking you to have a DSB check and giving you a few classes.
To make a substantive impact, you'd need training in the schemes of work that the school uses, access to confidential student information, training in documenting progression/assessment/interventions, someone to supervise/mentor you, plus training in safeguarding, fire safety, school policies, CPD etc. I'm sure there are other things too.
This would require a significant amount of work from someone at the school and a financial impact in terms of providing training etc.
I'm not sure that a volunteer questioning received wisdom or questioning the daily data download would be all that helpful in all honesty, however well-intentioned it was.
Primary schools are often appreciate of adults who are happy to listen to children read though.