I have just started some Wider Opps singing in KS2, which means using my voice talking and singing for up to three and a half hours in a day.
I am a supply teacher anyway, and have fairly good vocal strategies. One of the reasons I went into this work is because when I have to cover all subjects with children I do not know, by the end of the day, - it's almost always end of the afternoon by the time I'm allowed to stick in some music!- my voice is fairly tired and I am not singing at my best.

Any tips, please, those of you who do this kind of thing full time?

I don't know if I can help but thought i'd reply as no-one else has!

My singing teacher used to teach in school. He used the same technique as he does with his singing, he places his voice higher up in his head which takes the pressure off the vocal chords and works more efficiently, he could be heard all over the high school but never tired of talking. If you want anymore help from that side of it feel free to pm me

Yes, it's all down to support and placement. My teacher students (if you see what I mean!) say that their voices do not get so tired after having had singing lessons.