What about a weather station like airpi.es
I'm a teacher and the beauty about this project is that you have electronics, programming and DT in building a box for it! You can then put it outside and let it run headless to record data 24/7. You could even pass it over the geographers to look after once you job of buildings it has done! The nice thing about this is that you can link it to your school website so parents can see a weather data stream and see what your club has been doing.
If I was you, I'd write a letter to your school PTFA (address it to the chair of the PTFA) and ask for the funds for your project. Explain all the things I said above about how parents will also be involved in seeing the data from home and they will like that!
You could ask for £100 and you would have enough for a pi, sensors and bit left over to build the box.