Hi - if the school expects you to use the book, use it, in the sense of follow the syllabus, and use at least the odd exercise if only by setting them for homework - and tell the students there's no choice. Be as open as possible to student wants when you can satisfy them, but don't put your relationship with the principal or even your job on the line. Which of course doesn't mean that you can't discuss the situation with the school and suggest an alternative - decide what you want to propose before you raise the subject so that you show that you not only have good reasons for saying "no" to the book, but know what you want to put in its place. And make that another book - at least as the basis of a course. The students don't necessarily have to have it, but it will give you something to follow. That means you don't need to decide what to teach and worry about planning a coherent syllabus - that's done for you.
Then, whichever book you're using, you just have to decide
how to teach whatever the next topic is. I have to say that with 60 students, you're in an enormously difficult situation, and I'm not surprised that other teachers just let them play on the computer and watch videos. But hey - that means you've got computers and videos in the classroom. Why not build on that? There are loads of free materials on the net -some of them on the sites of people who contribute to this forum. If there are computers at hand, have a look at what's the next language item in the book that you're following, do a T-centred presentation and then let them do practice activities from the net. If the class are working autonomously it will give you a chance to circulate, monitor and help individuals - which you'll never be able to do with 60 people working in lockstep.
Then come back into lockstep for IW/PW/GW activities - which may be written or spoken drills, games, information gap activities or conversation, but always on the objective of the lesson.
Or if it's a topic based course, look on the net for a questionnaire on that topic, then some reading or listening work. Or a video on the same subject. There are plenty of ways you can use video which are productive didactically. There's a two part article
here and
here that describes some of them.
This constant change of focus will bring variety into the course and that in itself will add interest. But avoid long teacher-class stages - with the best will in the world you'll find it difficult to make that interesting for someone sat at the back of sixty others.
I'm presuming that they're at least teenagers if not older and intermediate level plus. But even if they're younger there's stuff on the web that you could use.
As regards the tiredness problem - start the lesson with an energising warmers, and always have some up your sleeve for if pace drops in the middle. Pace really is the key factor in determining whether a lesson is interesting or boring, and the more tired the students are to start with the more "peppy" the pace has to be to re-energise them.
I'm fishing in the dark here as without knowing your exact situation it's difficult to make concrete suggestions. If you want to follow up on anything in more detail pm me or go on with the thread. But as before - don't beat yourself up about it. You're in a hugely difficult situation and so far have done everything right.