Translation:Every pupil writes what the teacher says.
Should it be, "Ĉiu lernejano skribas tion, kion la instruisto diras"? Maybe I'm too used to that format so I use it too often.
It can. That's the correct long form but there's nothing wrong with leaving out the ti- part.
Note: one can only omit tio(n) in constructions with tio(n), kio(n) if they match in number and case (i.e., if both have an ‘n’ and/or ‘j’ or both do not). The same holds for other ti-, ki- constructions that can take the the accusative case and/or plural.
To add to that, you also can't omit ti- words in a ti-, ki- contruction when one of them has a preposition and the other doesn't, or when they have different prepositions.
Mi faras (tion), kion ŝi faras. (I do what she does)
Here tion can be omitted.
Mi faras tion, kio estas grava. (I do what is important)
Here tion can't be omitted. Kio doesn't have -n.
Mi iros (al tie), al kie ŝi iris. (I will go to where she went)
Here al tie can be omitted.
Mi iros al tie, kie li loĝas. (I will go to where he lives.)
Here al tie can't be omitted. Kie doesn't have al.
Mi iros al tio, sur kio estas banano. (I will go to that on which there is a banana.)
Here al tio can't be omitted. Different prepositions.
Ooh, I vaguely came across something about that somewhere, but totally forgot how it exactly was. Thanks for the explanation!