Q:

What is a nice word to describe someone who is never where they should be?

I had a math teacher in grades four through seven who was never in class.
What’s a nice word to describe his lack of presence? 

A:

Peripatetic - notionally means "wandering about" or walking or wandering but has overtones of Aristotelian teaching style - teaching while walking. Quite a fun term to use in this context. Wikipedia (link above) says 

" ... "peripatetic" is often used to mean itinerant, wandering, meandering, or walking about. After Aristotle's death, a legend arose that he was a "peripatetic" lecturer – that he walked about as he taught ..."

"Occasional" - while this is meant to mean that he is only sometimes your teacher and not always present at all classes , if he is meant to always be present then the meaning moves towards your required one. 
MIA / Missing in action.  
Itinerant - not usually used this way BUT conveys the intention when it is obvious that the usual meaning does not apply.  
Putative - implication is that they are meant to be doing the job, but ...

Mr Banks, our putative teacher, ...

" ..., When he puts in an appearance ..."
" ... on the occasions that he shows up ..."

A:

AWOL, astray, away, elsewhere, ghost, missing, no-show, vanished. 

You can characterize and caricature him by adding "Mr" in front of a name:

Professor Casper (the friendly ghost, and for a math teacher to be absent is very friendly),

or an adjective phrase:

Mr Out-of-Sight, Mr Unsubstantial, Mr Ethereal, Mr Incorporeal, My Invisible Math Teacher, Mr Wasn't There, Mr Illusory, Mr Imaginary, Mr Manquant (French for "missing"), Mr Missing. 

A:

That depends largely on where those individuals are, when they're not where they should be.
These aren't single words, but two expressions I might suggest would be:

chronically late (if they are simply tardy much of the time)
chronically lost (if they never seem to be in the right place at the right time)

The latter of those two can also be expressed using the more humorous navigationally challenged.

