As casual readers, we hardly ever make notes while we read. We may underline a sentence that strikes us, but mostly we're happy to let our brains do the work, and come up with ideas as our eyes keep taking the text in.
As a teacher though, you want to be prepared for anything your learners might come up with: they may have ideas and interpretations that surprise you, or spot things in the text you didn't - and ideally, you want to have thought of everything beforehand, so that you can face any idea, interpretation and question without fumbling for an answer or reaction. While you enjoy being surprised by readers' take on texts, you prefer being ready to deal with surprises - basically by knowing the text inside-out.
The solution is obvious: annotate your texts - take notes as you read - note down anything that occurs to you even if you have as yet no idea where these notes are taking you. The process of note-taking while reading is not designed to offer you all the answers right away: it is a cumulative process that starts delivering after you've read a fair bit of the text.
You can see below (poor) images of the notes I made while reading two novels for my classes (a YA classic I use in the B.Ed. English at the HvA - Hogeschool van Amsterdam - and a wonderfully quirky novel I use in the M.Ed. English, also at the HvA).
The most important point here is not the quality (or lack thereof) of the notes, it is in their very existence: as you can see, I simply make a note of whatever I find interesting, surprising, evocative and/or referential as I read. I write down the page number, and perhaps a couple of words - it could be a quote, words spoken that sound significant (even though I'm not sure yet how or why), or it could be an idea that occurs to me; typically in the form of a connection between that bit in the text and something else, in or out of the text.
The process continues throughout, but as you can also see in those images, after a while the notes start taking on a different character: I begin to refer back to earlier page-numbers, since what I observed earlier in the novel starts to find echoes in later parts of the same novel. So what I wrote down on page 40 now has an echo on page 75, and page 101: this, then, begins suggesting themes, or at least motifs - recurring ideas or concerns in the text.
For example, in The Giver below, you can see that I noted down, for pages 79/80: 'Physical punishment at school', simply because the narrator, on those pages explains that one can be severely beaten up for a transgression. At that stage, I didn't know - nor did I really think about - what that could mean in relation to the whole novel : it was simply striking enough for me to note it down. After all, physical punishments have been banned for while now, and the idea of hitting school children for minor transgressions strikes us now as very odd indeed.
Then you can see that on page 114 another type of physical punishment is mentioned: that of old people in the nursing home. Coming across that, I realise that this punishment echoes that of the children at school - something I noted down for pp.79/80. So now I can write down 'page 114' next to that earlier note, and note 'Old people' next to it. I now have a motif that emerges of itself: transgress and you will be physically punished: very young, young or old makes no difference. That in turn immediately begs the questions: who would do that? What type of society would accept that, and find it normal? What does it say about that society and those people?
What does that mean? I still don't know at his stage, but certainly an idea is beginning to emerge, and questions, too, for example:
How do you feel about beating up old people when they misbehave? Who does that, and why? What does it mean about e.g. emotional connections, social cohesion? Is beating someone up the best way to make them learn? Why would you use violence to teach someone social rules?
Obviously, I have no answer to those at that stage, nor do I need those answers: asking the question(s) is what matters right now. Once I've finished the novel I can start thinking about interpretations.
By the end of the novel, if you've taken good notes and have been consistent in your annotations, all your notes will start resonating with each other, and you will begin to see clearly ideas, motifs, themes and questions. That, in turn, will enable you to organise those notes in types of ideas, or types of questions, which automatically leads to interpretations.
Annotating your texts in that way takes time, yes, but it saves time too eventually. Once you've done that work, you can use that text for many years should you want to; you'll be prepared for pretty much any take/interpretation that readers may have; you can respond to any question about the text; you can also quickly find relevant quotes or episodes in the text. In short: you OWN the text, you make it yours so that you can use it to discuss and evaluate ideas rather than be stuck with factual questions or an anything-goes approach.