Can I end this sentence with "also" or "too"? Which one is right?

'Too', 'also' and 'as well' are quite often interchangeable. In this instance, they are, and can all be used at the end of your sentence.

However, if you have a preference not to end your sentences with 'also', try this:

I scheduled to stay after school with you today, but yesterday I was also assigned a detention for today.

However, I do think the sentence as a whole sounds a little awkward. From what I gather of the meaning, I would reword it to something like this:

I would've spent detention with you after school today but, yesterday, I was also given a detention.

In both cases the sentence itself is already horribly broken for the purpose of this question, so while the answer is easy (totally okay either way!), using this sentence to explain it is really hard. The problem is that there is no reading of the sentence in which ending it with "too", or "also", or putting too or also elsewhere in the sentence, results in a clear sentence. The sentence only makes sense if you remove too or also entirely, which makes it a bad example for this question.

More detail:

"I scheduled to stay after school with you today, but yesterday I was assigned a detention for today too."

What is "too" supposed to be referring to in this sentence? Are you saying that you received a detention yesterday for yesterday and also for today? Are you saying that I received detention for today and you received one for today as well? Are you saying that I received a detention for yesterday and you received one for today? Are you saying we both received detention on one or both days? If you are going to use too or also, you need to have something clear and specific for them to refer to as having taken place previously/as well/too/also. Out of context no reader could EVER tell what you meant by this sentence. In context it is still almost certainly unnecessarily confusing.

The sentence only makes sense without too or also, like this:

"I scheduled to stay after school with you today, but yesterday I was assigned a detention for today."

See, now I know what happened. You got detention for today. You or I may have gotten some other detention recently, but that relevant part is that I can't meet with you as scheduled today because yesterday I got a detention for today. Adding too or also just results in me having to puzzle out what prior detention, not defined in the sentence, you are referring to.

More importantly, since this is the only way to write it that makes any sense, it obviates the need for the word too or also, thus making the question itself pointless and this a horrible example to use for the purpose of asking the question you have asked.

That said, it is totally fine to end a sentence with too or also, as long as the sentence makes sense when you do so (the too or also has something to refer to!)