https://mobile.twitter.com/search?q=ノコ勉
&src=typed_query&f=live
https://mobile.twitter.com/search?q=のこ勉
&src=typed_query&f=live

https://www.google.co.jp/search?q=のこ勉
&num=20&client=ms-android-motorola-
rev2&prmd=ivn&source=lnms&tbm=bks&sa=X&ved
=2ahUKEwi75_HRm_7eAhWMXrwKHXfUDHQQ_AUoBnoE
CBcQBg&biw=360&bih=592
used in a book from 1992: "宿題をしなかったた
めにノコ勉をさせられて、少し遅くなってしまった。"

It might not be super common but it seems 
to have been around for some time and 
still sees at least some use.

@@ -6,0 +7,3 @@
+<k_ele>
+<keb>ノコ勉</keb>
+</k_ele>
@@ -8,0 +12,5 @@
+<re_restr>のこ勉</re_restr>
+</r_ele>
+<r_ele>
+<reb>ノコべん</reb>
+<re_restr>ノコ勉</re_restr>
@@ -12,0 +21,3 @@
+<misc>&abbr;</misc>
+<misc>&col;</misc>
+<s_inf>abbr. of 居残り勉強</s_inf>

http://kotonoha.cc/no/159678
20 or so responses, only 2 professing to 
not knowing the term

Might be (or have been) more common than 
what you can assess just from web results.

のこ勉	33
のこべん	59
残勉	126
居残り勉強	2624

I saw a site saying it was from 居残り勉強. I guess 残勉 could be ざんべん, as with 残業.
As for rejection, I agree it's not common, but if it's an emerging term I'd like to record it.

363 googits.

I presume のこ comes from 残る.
I think we should reject this. It's not at all common.

This is what I'm getting from WWW sites discussing the term. Any idea where the "のこ" is coming from?

@@ -12 +12,2 @@
-<gloss>???</gloss>
+<pos>&vs;</pos>
+<gloss>staying behind after school to do private study</gloss>