English question, please help.?

Grammatically you may use either. If you are talking about 1 subject you would use the singular. If you are talking about several different subjects you would use the plural.
English people would not use achievement in this context.
We would say "These students often achieve low marks" or "These students are often low achievers"

both are correct.

No, these students often attain a low achievement. See a put the a meaning one. But achievement is like saying low in one area. Achievements meaning more then one, plural with the "S" on the end, on your second sentence since plural with the S no need for the a low. Get it

number 2 is rite

These and students are plural so, the proper word needs to be ahievements