Hmmmmm, this is really a thought provoking question. I think the heart of every teacher would LOVE that his/her students who were truly interested in the subject read ahead. Still, I am going to go out on a limb and make a matter-of-fact attempt at an honest answer here.
It's all about motive.
As a teacher of regular level and advanced level classes in high school, above-average students who read ahead often annoy me. (Gosh, I hate to admit that!) Often, the read-ahead students "ruin" particular plot points by planting non-intentional (intentional?) spoilers. Many times the students are reading ahead to look smarter than everyone else in the class. It is more distracting to the teacher and the rest of the class than it is to the student himself/herself.
As for an average student (apart from the often above-average students I mention above), I simply suggest doing precisely what the teacher asks him to do ON TIME and WELL. That's all I can ask. I'm very happy with any student willing to do that. I always give big rewards to hard workers.
Upon further reflection, my students who are truly struggling, I suggest coming to me after class for extra help on what we are currently reading, ... I don't suggest reading ahead and gaining excess info we aren't ready for yet.
Just as a fun example, think of what Hermione Granger was thought of when Harry Potter first came out. She ALWAYS read ahead and was ALWAYS thought of as an "insufferable know-it-all" by students and teachers (well, Snape at least) alike. Sad, I know. She spent the entire book series combating that original impression.
Therefore, theoretically reading ahead is wonderful, but realistically I'm not sure it's such a good thing at the high school level. College is a different story, ...
It depends why the student is reading ahead. If students read ahead because they know it will help them understand the material better when it is covered, and then they pay attention when that material is covered, then reading ahead is a good thing. If, however, they read ahead, and they think they know the material so their attention is not as focused, then it is not a good thing.
I find that students who read ahead and then take time to review the material that was assigned for that particular class tend to do better than those who don't read ahead. Students who read ahead AND REVIEW have a better understanding of the broader significance of events, characters, symbols, motifs. However, I find that those who don't review tend to do worse on quizzes. It is so easy to forget details or to confuse the chronology of events.
When teaching a novel, I try to reread along with my class. If I reread too far ahead, I know I will refer to events that my students have not yet read or might overlook important events in that particular reading assignment in the discussion. But I don't teach a novel that I haven't read first. I feel that I should serve somewhat as a guide, pointing out significant lines, developments--something I can't do if I haven't read the novel.
With an assigned work in literature, readers often find that if they read ahead, and especially if the readers finish the work, then they can return to where the class is and really better understand the implications of the comprehension questions,etc. since they can perceive the work as a whole.
With certain authors, it is very helpful to read ahead, for the reader then learns the connection among characters. For instance, with an author such as Charles Dickens, there are often dual roles that character play, or they have more than one identity which is not revealed until the end of the novel.
I think it's good to read ahead if you know that you might not have time to do your reading later. However, if you don't need to, I think it is better not to read ahead. If you read ahead, you might have problems doing the questions posed to you by your teacher. If you already know what is going to happen, it might be hard for you to think about what has happened so far in the book without being prejudiced by what you know will happen next.
So read ahead if you have to, but if you don't, I would recommend that you just go with what is assigned.