Hi Gruditch,
Projects like this are heartening really, especially on the back of the other local doom and gloom stories that we have touched upon recently. It kind of restores ones faith a little!
I would be inclined to contact the county wildlife trust for their advice. I know that back-in-the-day creating wild areas was as simple as over-sowing with a commercially produced 'wild flower seed mixture', but in these more enlightened times this method is not necessarily considered best practise. The reason being that such commercial mixes often contain seed that is a commercial fodder crop or similar and is not necessarily british in origin. Such plants can have an adverse affect upon local flora once the seed spreads.
A far preferable method (although more time consuming to establish) is to scatter hay cut from a local species rich area (of similar soil type etc.) over ground that is suitably prepared (eg:chain harrow). That way the resulting species mixture is as close to 'natural' as possible and compatible with the local flora and environment as a whole.
Individual plants of specific species could be grown from carefully sourced seed and transplanted into the 'meadow', this in turn could also provide a good classroom project (if todays litigious society will allow school children to undertake such a high risk exercise

).
Lastly, if considering planting a tree or two it would again be prudent to try to ensure that the tree has been grown from native stock. It's ironic that so many of the recent well meaning (but ultimately fairly pointless) 'woodland creation' projects have used trees of continental stock that wouldn't be found in native woodland.
What a great idea though, your local primary school are to be commended, and a project like this is a great hands-on way for young kids to get involved with conservation.
Let us know how it goes...
Felix.