The ones posted above are solid units, as are Motorola units in general. These may or may not be sufficient depending on the size of the school, particulary if you're in a large secondary. The PMR446 standard is quite basic and limited, and the RF performance of PMR radios will be pretty much identical. The only real difference will be mic/speaker quality and battery quality. Due to this, it might be worth purchasing two cheap units such as these:
https://www.amazon.co.uk/Binatone-Latitude-Radio-Channel-Cancelling/dp/B097CFGLK3/ref=sr_1_5?crid=3BQBGWVRV223T&keywords=pmr+radio&qid=1676974564&sprefix=pmr+radio%2Caps%2C77&sr=8-5
Have two of you test these around different areas of the school - this is essentially the only way to truly know if the range will be adequate. If these work, all good - you can use these alongside any other PMR radio and they will be compatible. If not, you can purcahase a license, which will allow you to use higher powered radios, more likely to have adequate range. If I recall, this is around £70 for 5 years, and gives you access to a range of frequencies to use.
One word of warning - the PMR radios can of course be used by anybody, so individuals will be able to listen into your communications if they wished to - something to keep in mind
