The simplest way to measure soil pH is with a soil test kit. These should be available at garden centres near where you live. Do a Google search on: Enlgland soil test kit.
Alternatively, you can probably collect soil samples, mix them in distilled water, or else water that you have predetermined to have a pH of 7, put about one measure of soil to five measures of water. Then you can use simple pH paper probably available in your school science lab supply room. Be sure to collect about 5 samples per field, test them all, and average the results.
Finally, you can use a pH meter, if you have one available in your school. Same as above but use the meter probes instead of test paper strips.
Solutions of different pHs are generally made using buffered salts. There are many recipes on the web for making buffer solution, and you can even buy prepackaged sets of buffer salts from chemical supply houses. There is a great deal on the web about this. Use www.google.com and type in "making buffer solutions".
Any seed can be used in your experiment. I suggest quick growing commonly available seeds such as bean, radish, pea or corn. There are a lot of experiments posted on the web like this. Use google and type in "soil ph experiment".