You don't mention if you know the identity of the author, so I've assumed you don't.
The general rule for works of European origin is that copyright lasts for seventy years after the death of the author. Where the author is anonymous, we can apply the provisions of section 57
of the Copyright Designs and Patents Act 1988. Note that the Act talks about reasonable assumptions. So let's assume the teacher was no younger than 20 when she wrote the diary, and she lived for the average lifetime for a woman in the first half of the twentieth century - 64.7 years - she would have died in 1952, to which we must add 70 years, meaning that copyright will last until the end of 2022. Obviously there are some big assumptions here. For instance if you can find evidence in the diary that she might have been, for example, 24 when she wrote the diary, that could mean that copyright can be assumed to end at the end of this year.
Unfortunately, because the diary has not previously been published, we can't take advantage of section 12(3)
which also applies to works where the identity of the author is not known.
I have no idea whether you would like to publish this diary. If you do, you don't need to wait until 2023 (or earlier if you the necessary evidence she was older than 20). You can apply for an orphan works licence
through the UK IPO. However, if you decide to go down that route, you need to be aware that this type of licence only applies within the UK, meaning that, if it is your intention to publish, you would need to take reasonable steps to prevent the published diary becoming available outside the UK.
Obviously. if you do know the name of the author, much of the earlier stuff won't apply. However you can still apply for an orphan works licence if it proves impossible to locate any heirs or ascertain her actual date of death.