My son goes to Slough Grammar, so I know it better than the others. It is a language college but it has recently obtained the additional specialism of science that requires good standards in maths as well.
Herschel is a technology college which means that it is also strong in maths and sciences. Langley is a Maths and Computing specialist.
http://www.standards.dfes.gov.uk/specia ... ?version=1
Unfortunately, there has been little (I should say none) feedback from parents with first hand information about Langley or Herschel on the forum so far. Hope you get lucky...