Why where they so much more skilled than today's schoolchildren?
replies(6):
OED https://www.oed.com/dictionary/hen_n1?tab=forms#1717329 says "hean" was never a standard spelling of "hen". 350 years ago would be the late 1600s when there were "hen" and "henn" and "henne". (I don't know exactly when in the 1600s the latter two stopped being used; 350 years ago might actually be too late for those.)
On the other hand, the idea that for every word there is a single Correct spelling, as opposed to "write it however you like so long as it's clear to the reader", wasn't so well established in the late 1600s. But I think most 17th-century English folks would have regarded "hean" as wrong, not merely unusual.
(The article itself calls "hean" a misspelling, though of course that doesn't prove much.)