With respect to our proposed motto

Below is the reply I received from the Head of Classics at Malvern College....... I had forwarded Malcolm's attempted translation of Piss with the big dogs in the tall weeds...........

"To peese - in high grass - with/when - the dogs - a big one"
What sort of blether is this, Ricketts? I knew you weren't paying attention when we did agreement and cases! Obviously too much prep time spent in the nets, rather than learning Latin vocab and case endings!
Pissare: surely pisare - very late Latin - and why the infinitive? You want the imperative!! Hence: pisa (singular) pisate (plural).
in alta herba: well, at least you got something almost right. But why singular? And didn't we tell you adjectives follow the noun they describe? Surely in herbis altis.
cum takes the Ablative Case for goodness sake!
And the Ablative plural of canis - and I thought we had been through the 3rd Declension a 1000 times! - is canibus. No, not cannabis, you clown!
Stop fiddling with your glasses and pay attention
magnus: this is heart-breaking, Ricketts. Adjectives have to agree with the nouns they describe. So not magnus (Nom, masc, sing) but magnis (Abl, masc, plur).
So, write out a hundred times:
pisate in herbis altis cum canibus magnis
and hand it in at Break tomorrow!
Actually, Charlie, not a bad crack at it - after all those years!
If you want it in more Classical Latin, try this:
mingite in herbis altis cum canibus magnis! = [you lot, go and] urinate in the tall weeds with the dogs!
Or perhaps you might want to say:
mingimus in herbis altis sicut canes magni = we urinate in the tall weeds just like the big dogs [do]