Even so early as the beginning of the fifth century, St. Augustine
tells us that the vowel-quantities, which it was necessary to learn
in order to write verse correctly, were not observed in speech. The
Latin-speaking schoolboy had to learn them in much the same fashion as
did the English schoolboy of the nineteenth century.

It is interesting to observe that, while the English scholars of
the tenth century pronounced their Latin in the manner which their
ancestors had learned from the continental missionaries, the tradition
of the ancient vowel-quantities still survived (to some extent at
least) among their British neighbours, whose knowledge of Latin was an
inheritance from the days of Roman rule. On this point the following
passage from the preface to Ælfric's Latin Grammar (written for
English schoolboys about A.D. 1000) is instructive:--

Miror ualde quare multi corripiunt sillabas in prosa quae in
metro breues sunt, cum prosa absoluta sit a lege metri; sicut
pronuntiant _pater_ brittonice et _malus_ et similia, quae in
metro habentur breues. Mihi tamen uidetur melius inuocare Deum
Patrem honorifice producta sillaba quam brittonice corripere,
quia nec Deus arti grammaticae subiciendus est.

The British contagion of which Ælfric here complains had no permanent
effect. For after the Norman Conquest English boys learned their Latin
from teachers whose ordinary language was French. For a time, they
were not usually taught to write or read English, but only French
and Latin; so that the Englishmen who attempted to write their native
language did so in a phonetic orthography on a French basis. The
higher classes in England, all through the thirteenth century, had two
native languages, English and French.