I am working on an economics project to do with global warming, how much energy can be saved approximately in a school?

The syntax of your question is kind of faulty. Is the project on saving energy or on global warming or on some kind of combination? To know how much energy your school can save you really need to identify its energy usage and how that energy is created. In the southeast, we get most of our electric power from coal power plants. In other areas you might be using a hydroelectric power plant or nuclear power. Each is different. Also, is your school using incandescent bulbs or fluorescent bulbs? Is your school involved in a recycling program? Does your school use rain barrel water collection? Do you have solar panels on the roof? Are your school busses running on bio-diesel? There are all kinds of things your school could be doing…where will you start?

I am in Spain and it is on efficiency and advantages using solarpannels…

This site has an energy savings calculator for solar panels that might help you.