I can give you a few partial answers at least. The first, is that English generally wasn't taught in schools. For a long time, going to school meant learning to read and write, but not to read and write English, instead students learnt Latin, French, and so on. This would be pretty prevalent from the Norman invasion for several hundred years.

Another answer is that English was largely standardized by the King James Bible, and up until that point people just learnt whatever spelling conventions were common in their region, and for their teachers.

Yet another answer is that Old English, i.e. Beowulf / Alfred the Great style old appears to have been written quite phonetically, i.e. learning to read and write wasn't nearly as difficult as it is today, so kids or adults who decided to learn to read and write would basically go through copying out the letters, saying the sounds, and then would jump right in to sounding out words, with the full expectation that they now knew how to read (English, at least).

After the Norman invasion, with the introduction of thousands of French words, and the addition of thousands more Greek origin words via Wycliffe's early English bible and the many other changes that were taking place, well after all that English was a little bit tougher to learn how to read, and likely involved a good deal more copying, and a lot more hassle!

Yet another answer is many/most people didn't learn how to read in school, they learnt from parents/siblings/priests and so on.

My final answer is that as standardization followed hot on the heels of printing, and in particular was largely a result of the printing of the bible, the biggest difference following standardization was the availability of printed curriculum and the ubiquity of the bible as a part of learning to read and write, especially in more rural areas. Of course very quickly there were thousands and thousands of easily available texts to use. And remember that even printing didn't result in just one standardization - Scottish English had it's own standardized form, which persisted for a long time, really again only losing ground due to the popularity of the KJV bible and James's takeover of England - in fact it still persists, just not to the same level.