For the SCA types amongst us, I am being roped into doing a one hour class on how to get started in home brewing at this year's Pennsic War..... Not finalized yet, but this could be the class description.....
To be fair to your audience, you should let them know that:
1) It's just about impossible to make medieval-style beer using modern ingredients.
2) Just about all beer styles, including some styles we think of as being extremely traditional, are fairly modern.
If you want to give your students a sense of what medieval beer really tasted like, consider whipping up a batch of Kvass, Sathi or Gottlandsdrikke, or else bringing some bottles of straight lambic.
For a reasonably authentic 7th to 16th century Western European style ale, you'd need to use undermodified, smoked amber to brown malt (no extremely pale malt or roasted malts - those are 18th century inventions). Perhaps something like Weyermann Rauchmalz, or a very lightly toasted and smoked English mild malt. Smoke character should be as neutral as possible - beechwood or alder is preferred.
You are allowed to use malt and grains other than barley in your beer. Some period recipes include oat malt, wheat malt, rye malt as well as adjuncts such as peas and buckwheat.
No hops, unless you're going for a late medieval German or Flemish hop beer. Instead, use your favorite blend of gruit; bog myrtle, yarrow, heather, mugwort, wild rosemary, etc.
If you go with a hopped beer (as opposed to a gruit ale), use the most traditional, lowest alpha varieties you can find. Hallertauer Mittelfrueh probably dates to period, as do some of the precursors of Fuggles. For extra authenticity, pack your hops as tightly into a burlap bag as you can, but store them at room temperature for a few months before you use them. Expect alpha acid losses of 25-50% depending on age and storage temperatures.
Mash is either single infusion or decoction, unless you want to get funky and do a steinbier.
No crash-cooling allowed.
Fermentation is either open or in a barrel, with wild fermentation since the beer is left to cool in the open. You are allowed to use the yeast cake or the krausen from another batch of beer, though.