It doesn't sound to me like she's saying that teaching is the most tiring job ever and trying to do some sort of one-upmanship on nurses, doctors, oil rig workers, or what you or your other friends may do.
It sounds like she is saying only other teachers would understand how tiring their job is. There's a common perspective/assumption that it's a short work day with long holidays and therefore an easy option - which it is most definitely not. Think of how tiring it is to look after one, two or a handful of children. Now change that to 30+, add in catering for a range of abilities and needs, discipline problems without the range of options available to parents, plus marking, lesson planning, meetings, parents evenings, after school activities and perhaps before school and lunch activities too.
Even if she is saying her job is the most tiring ever, maybe - since she's your friend - she needs you to be supportive rather than finding her problems mildly irritating? If someone has a newborn baby that never sleeps and they're shattered, it's no help to them that people with 4 children have it harder. Perhaps the same with this. Being a doctor may be more objectively longer hours and more tiring, but your friend is struggling with the job she does and has trained a long time for. Even if you're not telling her you find what she's saying mildly irritating, are you sure this isn't coming across so that on top of being knackered she's also worrying that she's about her friends not thinking her problems are sufficiently important?
(btw, not a teacher but I know lots of them)