"Why do you teachers make us translate such long, difficult and ugly sentences every day?"
Translation:Waarom moeten wij van jullie leraren en leraressen elke dag zulke lange, moeilijke en lelijke zinnen vertalen?
does not make sense to me. either it should be "leraren" of "leraressen". as in English it is only "teachers" once only. please advice.
I presume you are referring to translating the sentence from Dutch to English?
As you are addressing a group of teachers (both male and female) you can translate it in English to "teachers" as in English generally does not distinguish between male and female teachers, whereas in Dutch you can.
The other way around it is the same, perhaps it depends on the context, but if the group of teachers contains both male and female teachers you can use "leraren en leraressen". Since no context is provided on the group of teachers. All options are accepted "leraren/leraressen/leraren en leraressen".
Zouden wij de leraren niet met u moeten aanspreken?
You can use u in plural, but it isn't always used. If you are talking to multipe people who you would address with 'u', jullie is considered formal as well.