Translation:He was my student in the fifth grade.
In England the fifth year of secondary education is called Year 11. Year 0 is "Reception" which children are in the year in which they become five years old. So Year 11 students will be rising 16 and usually take their National General Certificate of Secondary Education before going into Years 12 and 13 called a Sixth Form (often in a distinct unit called a Sixth Form College or in a totally separate institution called a College of Further Education. This sentence is going to mean something different for students across the world.
To be clear, I believe the "fifth grade" here refers to your "Year 5" in which students are around 10 years old. There is preschool and kindergarten which are our "Year 0" but once a student starts in the school system at age 5/6, it is the first grade. "Year 11", when students are 16/17, refers to the "eleventh grade". There are definitely still differences, for instance I have never heard of a "Sixth Form College" here, but your "Years" are parallel to our "grades".
For people across the world, "fifth grade" might have a different name, but I think there's always an equivalent. For instance in my country, fifth grade is "tahun lima". Depending on what language you're trying to communicate in, I think you need to be a little familiar with that country's schooling system to explain something related to it. In this case, since duolingo is centered around American English, I guess it expects you to be familiar with the terms, but I don't think it's that's complicated.
What age is a quinta serie, because as it stands, it doesn't mean anything to me.
The problem I have with these kinds of translations (whether it's Portuguese or any other language) is that it's difficult to make the equivalents of what grade/series/year/whatever we're talking about without context. They result in discussions (which can be interesting) about how these things work in different countries, but they're not particularly helpful for these one-sentence Duo translations. If this were part of a long document that we had to translate, we would need to research about how the education system works in the country the document is about and whose language we're translating into English, but this is Duolingo: there is no context and it shouldn't require extensive research into the educational system in Brazil (or Portugal, or the U.S. or any other country.) We have no idea whether the "speaker" here is a Portuguese-speaker in Brazil or Portugal, or any other country. I'd like to see this kind of question taken out altogether or simply reframed to explain that when one is in the X serie in Brazil, one is Y years old, or the child begins school in the X Serie at Y years of age. That way we could both translate and learn something and possibly even retain the word "series" (although I'm not even sure about that.) Just a suggestion but, for every language course I've taken in Duolingo, the arguments come up about just what this really means and which context we're using. What is the fifth grade in the U.S. is grade 5 in Canada and the child is 10 years old at the beginning of the school year (and it's a bunch of different things in other English-speaking countries as others have said), so more allowance needs to be made for accepted responses, or they need to accept the word "series". Sorry for the long-winded comment and I realize Duo has its limitations (they use the same expressions in different languages I've studied and the same arguments come up). I've made a similar suggestion in "report" but am not sure how much control the mods or anyone has in making changes.