Controlling boss/constructive dismissal?

Potted history - employed 2 years this Sept, in following March boss husband terminally ill she took time off and I was asked to step up - I did not get paid her salary but halfway between. Her husband passed in Sept she came back to work. Since then and because I learnt the job very quickly I do a % of her work and spend a lot of time twiddling thumbs as my job is the assistant at a level where I should be told what to do. I now don't because I know what needs doing (I work in Exams so its cyclical). I initially requested a meeting with HR re my workload in April - this happened in June. In between I have assisted other depots but have been told by my boss its because I am too efficient and I should not mention it as she may lose me to half time. After the HR meeting I got more work - its above my level but nothing I cannot do so did it. On Thursday after finishing the task for the 4th time my boss changed the goal posts again, I asked her to clarify exactly what it was she was asking make as it had changed since the initial request. Her reply was to blame her boss and then email my reply to her friend at work. Unfortunately I walked into her office to find the colleague with the email copy which she quickly tried to hide so I calmly and politely told my boss if she had an issue I would appreciate it if she spoke to me and not anyone else. I then spoke to HR again. Cue huge teary episode in which she was allowed to go home. She had a meeting again on Friday with her boss and again was allowed to go home though they didn't actually tell me just assumed I would do her work. I am an Exams Assistant she is the boss so you can guess how busy it is at the moment. I will add she completely blanked me before she went home on three occasions, her two friends did the same too which I find truly pathetic. The school however do not seem to appreciate the effect this is having on me - spend most of my day demotivated sitting doing nothing or I do her work and dont get paid for it. And now what I can only describe as bullying because I have 'dared' to confront her. Any thoughts on how to deal with this would be appreciated - I did post in employment and 'leave your job' is not really an option.