My teachers, like yours, were very definite on the use of ~ise as a suffix not ~ize. Until now, I had not really questionned what they said. I find it interesting that both are grammatically correct, but common practice has made the standard to which we adhere.
Last term our head of English broke her wrist at a critical point in the report-writing cycle. I stepped in as typist so that she could dictate the reports. I'm sure it took twice as long as we ended up debating the grammatical accuracy of what was being written. I'm blaming her, she kept asking me if I thought it was correct. Normally, I would defer to her; her written English is well nigh perfect.