Why de langste and not het langst ...?
I don't really know, but I do feel it's because of the "van". Everytime you see van after a superlative, you gotta use the form "de +ste".
we say preferably: 'de grootste van de klas'
It does sound a little funny if you say longest...
I always say 'langste'. (Native Dutch speaker.)
In German as well one translates tall as "lang" and not "gross", for size (NL: groot, D: gross) could go in any dimension.
Shouldn't it be 'in class' instead of 'in the class'
No, you need to include the.
"Van" usually means "from". When reffering to the English word "in", Duo usually teaches us that the Dutch word is also "in". Why is "in" also a translation for "van"?
My understanding is that "van" can also mean "of". Prepositions don't translate one-to-one between English and Dutch.