I've worn full-weight chain and plate reconstruction items while running around for a full day, and I'm not physically fit at all - I'd say that a random geeky 12 year old boy would be easily able to wear an armor suit, the main wizard-combat problems being getting winded very, very quickly if running (so they couldn't rush in the same way as Draco's troops did), and slightly slowed down arm movement, which might hinder combat spellcasting. It is not said how long the battles are - if they are less than an hour, then there shouldn't be any serious hindrances; if longer then the boys would probably want to sit down and rest occasionally or use some magic to lighten the load.
This. I've also worn multiple layers of armor, and something that's heavy to lift with your hands becomes much easier to handle when you're supporting it with your shoulders/body. If we extrapolate from harry, they transfigured the armor into existence, so it could be even lighter than average armor in any case.
Update: Discussion has moved on to a new thread.
The load more comments links are getting annoying (at least if you're not logged in), so it's time for a new Harry Potter and the Methods of Rationality discussion thread. We're also approaching the traditional 500-comment mark, but I think that hidden comments provide more appropriate joints to carve these threads at. So as of chapter 67, this is the place to share your thoughts about Eliezer Yudkowsky's Harry Potter fanfic.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six. The fanfiction.net author page is the central author-controlled HPMOR clearinghouse with links to the RSS feed, pdf version, TV Tropes pages, fan art, and more, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: