I had exactly this situation years ago and ended up in a massive argument with the Head - he turned out to be a nasty piece of work if you didn't just do as you were told as well.
It is a really grey area and there's not right or wrong answer, all you can do is discuss it with the Head and come to an agreement that you're both happy with.
The situation that arose with my Head, was because he insisted that I had to be available on all of those afternoons, and HE would decide, sometimes as late as on the day itself, whether I should attend or not. This was ridiculous for me, because it meant I would have to pay for childcare for all of those afternoons, even though most of the time I wouldn't actually need it.
I did arrange childcare at first, but the first few times, he told me I wasn't needed, on the morning of the meeting, so too late to cancel the childcare. This really pissed me off, so I stopped booking childcare, but then one morning, he told me I was required to attend the meeting, and obviously I couldn't attend - he went absolutely berserk and tried to put me on a disciplinary, which fortunately was immediately thrown out by HR, who thought he was being ridiculous.
In the end, the ONLY meeting he wanted me to attend over the whole year, was that one, but he'd wanted me to make myself available for every one of those afternoons 'just in case'. It was basically to cover himself because he couldn't be bothered to plan in advance what each meeting or training session was about.
It was ultimately all about control on his part - he wanted to make sure that I would jump whenever he asked me to jump.
Hopefully your Head is a bit more reasonable - my current Head is lovely and would happily sit down and discuss it like a pair of adults, and come to a mutually agreeable arrangement. My previous Head wouldn't even let me in his office to discuss it - I tried to arrange a meeting loads of times, but he just ignored emails and would not see me in his office - claimed he was busy.