is it okay to have lesbian sex with my ex school teacher

Well first, the idea of getting someone drunk in order to have sex with them is pretty wrong. If someone isn't willing and ready and comfortable to do something with you sober, then intentionally dulling down their decision making abilities and then taking advantage is pretty low.

Second, how recently was this person your teacher? If we're talking in the past year or two, I'd suggest staying away altogether. And additionally, what sort of relationship/connection do you have with her now? If you still see her as an authority figure, and/or if there's a major age difference, then the relationship is highly likely to be inherently unhealthy. A healthy relationship is built on both parties being on a pretty even playing field (mentally, emotionally, financially, stage of life) and has balance to it. If this doesn't, then it is likely to have problems.

So perhaps you could start by just having a conversation with her. See if it's something she's even interested in. And if she is, think about what I said above and whether this could be healthy for you, or if you'd still be seeing her in a superior/caretaking role. And finally, if she isn't interested, then let it go. Trying to manipulate her into starting a relationship is wrong on many different levels.