I learned that desert just has one s and you’d only want to go there once and that dessert has two s’s because dessert is good and you want two of them.
Connecticut = “Connect” “I” “Cut”
Together = “To” “Get” “Her”
Separate – Caught like a rat in a trap (from the Alice and Jerry books).
My favorite was not in school, but from an old AP style book:
burro – an ass
burrow – a hole in the ground
As a reporter, you’re supposed to know the difference.
tomorrow = tom or row
beautiful = be a utiful (whatever, it works for me)
how about, recess is necessary for hungry pupils.
A rat in the house might eat the ice cream. arithmetic
I am 31 years old and I admit I still use this sometimes.
Needscoffee, the phosphorus one is because the kids always want to spell it “phosphorous”- they don’t seem to have any problems with the ph parts, but they always try to spell it -ous.
Cool.
I never heard of the eight exceptions.
My Very Engenious Mother Just Served Us Nine Pumpkins.
That one got blown out of the water a couple of years ago. I cried.
(planets)
what’s ‘Engenious’? anyway, you could have her serve nuggets instead.
Naughty Elephants Splash Water
(compass points)
Cat and beer helped spelling. I’ll remember the Naughty Elephants one for my nephew. Very cool.
Will also change that to Nuggets for nephew, thank you 
How will fixing the spelling help? Mercury, Venus, Irth? I’m confused.
I learnt this as “My Very Easy Memory Jingle Seems Useful Naming Planets”. Maybe you could tweak it to "My very easy memory jingle seems useful, Nigel (or whatever name starting with ‘N’). And awkward sentence, but it sticks in the brain surprisingly well.
You might scream EEEK when you walk through a cEmEtEry
A Red Indian Thought He Might Eat Turnips In Church - Arithmetic
I just signed in for the first time in god knows how long to tell you I like…
One coffee with two sugars. That is all.
My Very Educated Mother Just Sent Us Nine Pizza-pies.
I learned that at a planetarium when I was little. I still remember the tune.
I suppose I could change it to My Very Educated Mother Just Sent Us Nine Pizza-pies, Eggplant Chorizo. Kuiper.
hmm, how about - My Very Excitable Mother Just Screamed ‘Ugh! No!’.
Em-eye-crooked letter, crooked letter-eye-crooked letter, crooked letter-eye-hunchback, hunchback-eye!
Much more fun to chant than to actually help remembering how to spell it.
(And the related riddle/joke; Mississippi is a hard word to spell: spell it.)
Wait, isn’t Phosphorous pertaining to Phosphorus?