buebug, hi, are you a US Citizen? If so, have you ever worked in Canada? If you have a reciprocal Social Agreement between the UK and Canada might allow you to claim Child Benefit, notwithstanding the No Recourse to Public Funds restriction on your visa.
Whilst you mention the problem with Child Benefit, you could well have the same problem with Tax Credits.
Are there any allowances for teachers or for single parents?
Certainly nothing for teachers, apart from something in the Double Taxation Agreement between the UK and the US. How long are you going to be teaching in the UK?
Single parents? No, nothing, an allowance called Additional Person Allowance was abolished a few years ago. Tax Credits are the way that money is now handed out, but you are probably excluded.