In 2016 Sega released a DVD compiling all of the episodes of Hi Scoool SeHa girls and included a bonus episode the rest of the show is easy to find as it was on crunchyroll but the bonus episode was only on that DVD which never left Japan and from what I understand it was a bit expensive. I have done some looking but have ultimately found nothing, there is a youtube channel for the DVD with the only 2 clips of the episode that i have ever seen they have links that send you to the website of the DVD and then a link to the amazon page but they no longer sell it i have even tried yahoo japan auctions for a copy but still nothing. If anyone has anything on this episode it would help to get this episode on the internet.
I think you may still be able to find the DVD popping up from time to time in places like eBay or Japanese Auction sites, but, it's rare, though, i don't believe it was thaaat expensive of a release, but, certainly not cheap either, me and my friends have never been able to find the episode on the internet, so, i believe it is safe to still consider this one lost media :/
I too am looking for it, but, it seems not many people is interested in finding the episode as much as we are... Let's hope for the best though!
i do have some what of a lead when i was looking for anything for it i came across a translation of the opening and in the comments someone claims they have bought it and someone else is translating it. Bryce Johnson in the comments is who provided this info
i don't know where they plan to upload it but i am assuming he would link it in a comment on that video because he was speaking to someone else about it so i am hoping for that
the full episode has been posted it's not got subtitles but it is no live on youtube and backed up on to google drive www.youtube.com/watch?v=tXtdfxVViRA
the full episode has been posted it's not got subtitles but it is no live on youtube and backed up on to google drive www.youtube.com/watch?v=tXtdfxVViRA