Translation:The teacher is studying with the grandfather.
The Greek letter beta (β) is not the same as the German letter Eszett (ß). http://en.wikipedia.org/wiki/Eszett
Anyway, it's not quite true that it's generally accepted to replace the Eszett with "ss". It's "ß" after long vowels and diphthongs, and "ss" after short vowels. This distinction is quite important because it tells you how to pronounce the preceding vowel. If you can't type German letters on your device, it's okay to always use "ss". However, since they want you to learn the proper spelling, you're getting this "almost correct" message. I don't think it's too strict because you won't lose a heart if you don't use the Eszett.
@Soglio: As you don't write very often in ALLCAPS, I can hardly say it was in common use. And even if you had to do an ALLCAPS HEADLINE, most people/writers/journalists already used the SS. I think the 1996 reform buried a dead spelling. (They couldn't anticipate the advent of SHOUTING INTERNET TROLLS, so perhaps they killed it too early :) )
Ooops...apparently the Gothic font alphabet actually includes an eszett...but it looks just like a Gothic "s" smashed against a "z" (here's a contemporary street sign for "Schloßstraße" in Gothic font: http://en.wikipedia.org/wiki/File:Erfurt_Stra%C3%9Fe.jpg.) and some printers wrote it as "sz" when people started switching to the Roman font (which did not have an eszett symbol) in late 18th-early 19th century. (see http://en.wikipedia.org/wiki/%C3%9F.)
The "SZ" was still allowed until fairly recently in ALLCAPS WORDS as a capitalized version of the 'ß'. But with the 1996 spelling reform this (already very dated and rarely utilised) usage was also dropped. I guess there are probably still some street signs in caps around were it is used. You also find it in family names. Today you have to write STRASSE or (incredibly ugly!) STRAßE when shouting on the internet.
Honestly, you can use it interchangeably - "That is his Grandpapa", "that is his Grandpa", "that is his Opa", they all work. Whichever one you use is going to be determined more by your cultural heritage than anything else. I lived with a family whose grandparents were Oma and Opa, while mine were Grandma and Grandpa.