If you didn't sign a contract with a clause specifying that any materials you produced (i.e. had to and were expected to produce) become the property and copyright of your employer, who has more than amply recompensed you with the basic wage blah blah blah, then I believe that you retain the right of authorship and copyright over the materials. (I am not sure of the legality of these clauses, perhaps factors such as "royalties" would undermine them to some extent, but employers at least seem to like to think that this covers them against any future claims).
Your concern then in that case should not be whether the college moves to prevent you from using (how could they know, really?) or publishing (more likelihood of being discovered) the materials, but whether somebody at the college (Japanese personnel, or a future foreign teacher) appropriates the materials as their own and seeks to make a profit from them ahead of you (assuming that they are of an enviable standard already, which seems to be the case, given your enviasged plans for the materials).
You could try to openly (or not so openly) remove the archives from the school, but that could lead into precisely the sort of (legal) problems/grey areas that we all strive to avoid (and doubtless you don't want to appear a totally selfish and ungenerous person).
In an ideal world, of course, schools would employ and appropriately recompense dedicated materials writers beforehand (or at least buy a range of optimally useful/appropriate materials, on the advice of at least one informed and "frontline" teacher), but as we all know, they unfortunately do not.
My explicit experience of this has been confined to two companies, one a large and very fair foreign-managed EFL in Japan employment agency that had a clause similar to the one in my opening paragraph for
general teachers, which was probably more for silencing those lazy teachers who might have objected to
ever being asked to prepare anything by e.g. a Japanese team-teaching colleague (the company also advertises for dedicated FT lesson-plan writing postitions, which seem quite generously rewarded per plan completed); and a pretty useless/incompetent small Japanese-managed company that had no such clauses in its shoddily-written contract, which resulted in me working for an exploitative school where I was developing materials on pretty much a full-time basis in my own time
for other teachers (and, admittedly, they for me, not that this reciprocity justifies the system at the school at all) after teaching full-time all day (plus whatever other extra-curricular activities and miscellaneous duties, of which there were many).
http://www.eslcafe.com/forums/teacher/v ... 1966#11966 (go down to about halfway through the post for "My story").
That *beep* school was of the opinion that teachers should keep and depart with their materials, but I rather got the impression that this was because the school didn't think much of
any of the materials (and/or couldn't be bothered sifting through it all and collating/archiving it) than because of any pressing legal arguments or precedents. I made it quite clear to them that I had no objection to surrendering my "rights" to the materials,
if they would only look at them seriously, assess their possible future utility/worth in recycling, and start helping future teachers who would also be expecting at least SOME ready-to-use, previously successful plans and activity materials.
And even if a school were not in such dire need of materials for present and future teachers, I would have no real objection to the majority of my materials being appropriated by the school because 1) I am a generally helpful guy 2) I don't live for money 3) Most of my ideas have been thought of somewhere before, even if to me they appear strikingly original and valuable and 4) even if I have something quite unique, it is not always certain that others will recognize or appreciate the value of it, so I can probably rest easy that I'll be the first to publish it (if there is actually much money to be made from publishing - I rather suspect that a teacher's main source of income will likely remain from immediate (hopefully satisfied!) students. In any case, if a legal battle did develop, I am sure that the fact that I and the other "writer" both once worked at the same school would work in my favour as much as his or hers...and doubtless I would have improved and continued to add various individual touches to any activity to make it unique enough in the eyes of the law to not be an infringement of copyright; then, there is the whole question of sequencing of materialS, the shape of Course A is often so different from Course B that simple allegations of plagiarism would soon fail - I mean, if dictionary publishers can't "prove" plagiarism then who can?!).