Translation:Shy students generally receive lower grades.
Alunos tímidos geralmente recebem notas baixas. = Shy students generally receive low grades.
Because "more lower" is bad English, the superlative is already included in the word- low, lower, lowest
I exchanged "grades" with "marks", but was rejected by Duo. I think this is an error. Although grades are more common than marks in US English, it is the other way round in British English or in English spoken in other continents than North America.
Os alunos tímidos receberam as notas mais baixas. (The shy students received the lowest grades).
O menino vestido de azul é mais alto do que eu. (The boy dressed in blue is taller than me)
O menino vestido de azul é o mais alto da escola. (The boy dressed in blue is the tallest in the school)
See if you can translate these:
Os biscoitos da Dona Dalva são os mais gostosos que já comi.
Os biscoitos da Dona Dalva são mais gostosos do que os biscoitos da minha mãe.
Princess Dalva's Cookies are the most delicious cookies that I've ever eaten.
Princess Dalva's Cookies are more delicious than my mom's cookies
Does lower mean worse in Brazil/Portugal? We have grades from 1 to 5 in Czech schools (except for the universities where it is usually only 1 to 4), 1 being the best and 5 being a failure, so in this sense lower grades are better grades.
Without context, I would say lower just means lower. Portuguese has a word for worse (pior) so if you wanted to say something was worse, that's the word you'd use.
Because while it is an accurate translation, it is not correct because the meaning is lost.
In addition, the sentence gives context to allow proper translation; it is talking about students. Students receiving "lower notes" doesn't mean anything, "lower grades" makes more sense