As to attending uni, firstly consult your contract. if you are employed on a full time contract and your previous boss had done you a favour in adjusting your hours to accomodate uni without amending your contract to reflect this, then if your employer now needs to change this then (since that's what you signed up to in your contract of employment) you can't complain and if you do go against it, your employer could dismiss you for breach of contract
If your shifts or number of hours aren't defined and you are given things on an as and when basis, then you can say you can't make these particular shifts. But the employer at that point is within their rights to say that the needs of the business change and therefore people are required who can do these particular shifts. In that instance, you could be made redundant.
As to staff meetings, yes you should be paid for it. It's a work matter whic you are being called for - thereforeyou are paid for work. Simple as. Tell her to consult acas.co.uk or 08457 474747 for confirmation of the legal stance before she threatens disciplinary action.
So stand your ground on the staff meeting - politely but firmly. As vinrex says, you don't want to p*ss her off.
As to hours, if you're not in breach of your contract, just see what happens as to whether she can accommodate it. She may just be going through the new-boss-making-an-impression stage so it might just go away. Although if it really can't be worked around, you have to accept you could be out of a job through redundancy.