The reason for this is contextual (internet posts), not grammatical. You'll get more results if you remove the polite ending and/or the topic particle (try "来週学校に行かない").

Actually, since not wanting to go to school is a common sentiment, it can end up simplified into something with no particles at all: https://twitter.com/k_y02240/status/613690896916201472

I hope you don't find it nonsensical, though, I understood it just fine.

And checking ghits for plain forms shows:

"学校に行く" - 3,680,000

"学校へ行く" - 444,000

so I think the other way is actually the variant!

BTW, I feel like "学校に行く" - where に means "for/into" - has a sense of "going to school to go to class", but "学校へ行く" - where へ means "towards" - has a sense of "going to the school building as a physical place".

Here's a forum thread about it:

http://oshiete.goo.ne.jp/qa/5458812.html

It is neither nonsensical nor an error.