Unis look at your actual grades and also predicted grades. I'm not sure what you mean by 'average school grades'?
I do think it is possible for you to get onto an Economics course at Uni, but you will have to be realistic about the sort of ones you apply for. Economics is one of the most popular things to apply for, and basically all the places in the top 20 will ask for AAA. Its unlikely you are going to achieve such high grades so you need to have a look at other options.
I'd really recommend looking at the UCAS Course Search function in order to have a look at some places that may be viable options. If you want to apply for a few high grade courses thats fair enough, but you need to have some courses which ask for lower grades and will be a good back up option if you don't get in.