First off, since you will be commenting on the level of someone's education, you'll usually want to be cautious in the words you choose, since no one would like their level of intelligence to be made fun of! ;)
Anyways, since you are looking for a polite way to express this, I probably wouldn't lean in the direction of
inculto. These words aren't totally rude per se, but they might come off as being a little too blunt. You could probably get away using them in a joking or playful manner, but caution is still advised. Other words that could probably fall into this category might include
bruto (brute), and
necio (roughly: uneducated fool.)
falto(a) de educación would probably work, but it could also be interpreted in a way similar to
maleducado, that is
rude. It would really depend on the context.
In my humble opinion, I think that a good (and unambigious) way to express
the lack of education, would be to use an expression meaning
without. In this case the preposition
sin or a negative of the verb
tener work well. These patterns also allow you to specify the type of education a person lacks (e.g.
escuela secundaria (high school),
colegio (college), etc.)
- Ella está sin colegio.
- Ella no tiene (un título del) colegio.
She doesn't have a college education.
That more-or-less should cover all the basic and general expressions that would be used. In general, the same principles that you would use to choose how you'd express this in English will apply here. Just try to be sensitive and smile. Hope this helps! :)