The legislation has changed in relation to DBS checks

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/97875/leaflet-england-wales.pdf
http://www.legislation.gov.uk/ukpga/2006/47/schedule/4

In brief, your staff need to be able to have contact with children that could form a relationship. Therefore, a delivery/pick-up once a month for a 5 - 10 minute stopover is not deemed as contact, especially when there would be teachers supervising nearby. The new legislation is more for a hands-on role.

However, if your staff were on school premises for a considerable time - at least 4 or more days in 30 days, once a week and able to have contact with children, then that would be different. They have scaledback to what they call call common sense levels with regard to Standard and Enhanced checks.

Failing that, you can carry out Basic Disclosure Scotland checks or Access NI Basic checks. These are purely for UNSPENT convictions - a PNC check but it's better than nothing and you would be compliant.