You did not say what grade level you are teaching but it sounds like it is probably high school. It would be interesting and helpful to know what message you conveyed to the pupils when you conferenced with them and/or their parents. It sounds as if this is very frustrating for you and you may be feeling some pressure from above if about 25% of your students are refusing to do their work and therefore failing.
Something I found moderately successful when dealing with 6th graders follows. It may be totally unusable with high school students. I hope it helps or at least gives you some encouragement.
When I had students who refused to do assignments. I would talk with them individually or in small groups and ask them if they would like to pass the class. I usually got answers like, "I don't care. It's boring. What do I have to take English for anyway?" Unless I paid very close attention I would miss the fact that they did not answer my question. They tried to avoid a simple Yes or No when asked, "Would you like to pss this class?"
I just asked again, and again, and again if necessary. Most students will not, absolutely will not say "I would not like to pass this class." In fact, no student I ever taught said, "I would not like to pas this class." That is a start. I now had established that the class meant something to them. They had admitted it.
I would then lead the discussion in the direction of the pupil telling me what he/she would need to do to pass the class. I did not accept "I need to do my work." which was the usual answer. I insisted on more specificity. Here I found a number of differnt situations that had bearing on the student's attitude.
Some had at some time tried doing their work and for various reasons had been unsuccessful. As Kate said these students had found a way to avoid failure but saw success as elusive. For them, doing the work did not ensure passing the test.
Some had played around and failed because they simply completed too small a percentge of assignements to achieve a passing average. They believed they could pass easily but still did not do the work. This is much like the first group in that they too are avoiding failure by not trying to be successful. They could cling to the illusion that they really knew how to do it and would not be bothered with demonstrating their mastery.
A third group seemed clearly to be unable to understand the relationship between doing school work and their grades. When asked what they could do to pass the class ( which they have said they would like to do) they honestly cannot give an answer.
No matter what the situation was it seemed to me that I had to get every one of them to tell me something specific that they believed they could do to pass the class. I know that sometimes I told them to think about it over night and we would discuss it further the next day. In a big majority of the cases whatever they said they could do really would work. Most of them knew what they needed to do to pass the class. Those in the third group who honestly did not know what they needed to do usually just followed the lead of someone else and that was OK.
Once a student began to identify things he/she believed they could to do pass the class we discussed it and analyzed it and evaluated it and realy talked a lot about it. I did this to convey to the pupil that I thought his/her idea was important and workable and something I could accept. Pretty soon I had the pupil really arguing on behalf of his/her own idea and trying to convince me to let him/her do it. I always remained skeptical but eventually I reluctantly agreed that if he/she did what he/she said they woud do and did it well and could show me they had learned the information or skill or idea I would give them a passing grade.
It almost always worked. The kids now felt they had controlled their destinies. They had set up something they believed in. They would work for it.
If you try to do this, please do not give up if at first you do not succeed. It is hard to do. If you are indeed working with high school students they will be more difficult to lead than my 6th graders were. You have 14 years experience and you are a caring and devoted teacher. You can make it work. Good Luck!
Richard