They should be ashamed off themselves!
In the unlikely event that your union or GP can't help or won't want to take sick, I would formally appeal the refusal.
Something like this...
Dear Management/Board of Governors/Shower of Heartless Tossers,
I wish to formally appeal your refusal to sanction two days annual leave to attend my daughter's graduation.
As you know, I have been a loyal and hardworking member of staff for X number of years, thoroughly dedicated to both the school and students alike.
Unfortunately, thorough no fault of my own, I'm a stage four cancer patient, unsure of what the future will grant me.
While scary and distressing, I have done my absolute utmost to ensure that this diagnosis has had as little impact on my performance, attendence and passion for both my students and my profession, while I'm sure you will both appreciate and recognise.
Given the future uncertainty, attending an event like my daughter's graduation is not only of huge importance to me, but also to my daughter and my family.
As my diagnosis progresses, such attendence at key life events may not be possible.
I am appealing for understanding and compassion, in these extentuation circumstances, and ask that youreverse this decision and that you grant me two days compassionate level to attend.
Please put yourself in my position amd reconsider your position.
======================================
Document how absolutely heartless this decision maker has been and shame therm into co-operating by ensuring absolutely everyone involved and those with the authority to overturn this decision are aware of how you have been treated.