Hi I am a current year 13 who has just finished A-Levels, here is what I have to say ...
First argue your case and say to them that you were very close to the B and therefore it will be reasonable for them to give it to you.
Show them the entry requirements for the course you want to do and therefore the importance of having predicted grades that match or are higher then what the uni requires.
If you aren't any early applicant (so not applying to Oxbridge, medicine, dentistry or veterinary science), then when come back after summer you school should give you more tests to do. Revise for these to prove that you are able to get a B or even an A, A*. Even if you are an early applicant (like how I was this year) you still have to time to prove your self and boost your grade.
In the process don't get demoralised, if you're school is anything like mine a lot of people were able to get their grades raised by 1 or even 2 in some cases, but you have to put the work in! At the end of the day most schools don't won't want to hinder you from getting offers to the courses you want to do. Because at the end it is up to you to get the actual grades in order to be accepted.
God bless
I hope this helps xx
P.S. It is very easy to get caught up in predicted grades and how this affects your uni application, but make sure to also reflect on your mock and see what things in econ you are struggling with and need extra support. Iron out those issues you have with the AS content so that when you go into Year 2 you will be able to grasp the other content more easily xx