It depends. If the portrayal of the yandere girlfriend is done in an interesting and nuanced way, it could be appealing. But if it's overly stereotypical or one-dimensional, then no.
A gift from her in a yandere manga can have multiple meanings. It could be a sign of the yandere's obsessive love, an attempt to control the situation, or a way to create drama and tension in the story. Sometimes, it might even be a red herring to mislead the reader.
The teacher being here in the manga might be part of a planned storyline. It could be to introduce a conflict, provide support to the main characters, or to drive the plot forward in an unexpected way.
Well, such a girl is often obsessive and possessive. She might go to extreme lengths to keep the person she likes close to her, sometimes even using violence or threats.
It's possible that the artist or writer of the manga had a specific inspiration or idea involving your teacher. Maybe they wanted to incorporate a familiar figure to make the story more relatable or interesting. Or, it could be a random choice for some unexpected twist.
There are several options. One could be Comico, which often has a wide range of manga available. Another option is Webtoon, although it's not guaranteed that 'Me and My Yandere Girlfriends' will be there. Sometimes, specific fan forums or communities might also share links, but make sure to follow the rules and be cautious of potential copyright issues.