Do colleges look down on half days in your senior year?

Short answer

Yes.

Long answer:

As I mentioned earlier, colleges primarily look at your junior year academics when making a decision about you. But that doesn’t make your senior year unimportant at all.

Let’s put it this way. Say you’re an employer trying to decide who to hire. You look at one person’s application and see that he was a great worker at his current job, but you find out that once he had the prospect of being hired by you, he stopped working as hard and kind of turned a blind eye to things. The other person kept on working strong, even with a decent chance of being hired by you. Who would you hire?

Colleges are likely to do the same thing. Depending on how competitive the school you’re trying to get into is, having a shorter day your senior year can hurt you, especially depending on what you do with it.
If you shorten your school day to getting out at twelve noon and you decide to take really simple and easy classes, then that’s definitely not going to look good for you, even if you get all A’s. If you take four really hard classes and then go home, that’s still not as good as if you took four classes and then filled the spots with electives.

The only way I could see having a reduced day not being a penalty is if you do something productive with the time, like taking college classes, working a lot of hours at a job, community service, etc. But if you’re just going home to sleep and sit around, that’s not going to look great for you.

Again though, that depends on how competitive the school is. At smaller, less-competitive schools, it won’t be a big deal provided you’ve proven yourself up to that point. But if you’re trying to get into MIT or something, it will surely hurt you a lot more there.

That said, there are more important factors than how many classes you take. Junior year is huge, as well as extra-curricular activities. But taking fewer classes can reflect on you as being somewhat lazy. Be careful and make sure you’re not going to be hurting yourself by what you do.