"soll" should only be translated as "shall" if it could have been translated as "will", but if there is any doubt, then "should" expresses the obligation or probability without being 100% sure. "Should" is not just used as a conditional modal verb i.e. it is more than just "sollte". SelphieB's link lists "should" and "shall" as both being translations for "soll".
According to any English-German dictionary, should is a translation of sollen eg http://dict.tu-chemnitz.de/dings.cgi?lang=enservice=deenopterrors=0optpro=0query=sollen%5Ciservice=comment=email=
But the use of 'shall' varies between different countries, I think in the US and also here in Australia its hardly ever used these days except in highly formal contexts. So I think both should be accepted, I don't think you can say should is wrong.