Perhaps you will get the students on your side if you explain WHY you are correcting them. It comes down to this: do they want to sound uneducated to an English speaker? Chances are that they do NOT want that. Would they like to be laughed AT or WITH?
This is exactly why the first day of class we spend time on "how do you learn?" Once they know what kind of learner they are, they will do better in ALL classes, not just yours.
Get them to try to remember how they learned their native language (I assume it's Italian?) That is a Romance language and they are learning a Germanic language when they learn English.
There are really 3 modes of learning, however there are also combinations. 1) Do they learn orally? (easier for them) They they need to HEAR what they want to learn and repetition is a part of that. When they were first learning their mother tongue, didn't someone correct them and have them repeat until the sound was correct?
2) Do they learn visually? Then that means they must SEE what they want to learn. I have lots of study habits I did with my students but I was more fortunate perhaps because most of my students were learning their 3rd language. Once a student learns how to learn 1 different language, they have a "dual" track to which they can add a 3rd, 4th, etc.
3) Do they learn kinesthetically? That is how I learn and it is the most difficult mode. That student needs to hear, see and physically be involved, by tracint the letters, etc. A very effective way was to label everything in the house and when, for example, they saw a chair, it was no longer a chair but silla or chaise, depending upon the language they were learning. Most of my students spoke Korean at home, English in their other classes, but either Spanish or French with me. Then they would SEE the target word, SAY the target word, HEAR the target word, sit on "the target word" etc. until TPR or Total Physical Response was set.
I'll be happy to share lots of ideas with you, if you'd like. The faint at heart usually fled the first day when they discovered no English in my class. But who wants the students with a bad attitude? What you do is work on the attitude and when they learn that you only want the BEST for them, they will be on your side!
Shame on the headmaster who does NOT speak English and does not understand what you are doing!
Once upon a time our principal (who used to teach German and should have KNOWN what I was doing, grilled me with "WHAT are you doing, WHY are you doing, HOW do you do.....until I looked at him and said: "I KNOW what I'm doing; just get out of my way and let me do it!" He was actually a good friend and used to say that a lot to other students, winking at me!
Do not lose heart! Once the students understand what you are doing and why, they have to be with you!
Sra (aka Mme)