I translated "Es ist Schluss mit uns" as "You have closed with us".
I think that this sentence means that somebody has closed with us, and not that two people has finished their relationship, but my understanding of German is all but perfect unfortunately.
(Of course my wording is all but perfect, but I wanted to understand the exact meaning of this sentence.)
This is why I think anyone who is able to do so (i.e. intermediate/advanced and above) should study prepositions by translating entire sentences, rather than trying to translate the prepositions themselves: aus = out, from, of; bei = by, with, at; etc.
In german, like Menschen said, you can use mit or zwischen. In english, however, you would exclusively use between: "it's over between us" or just "we're over", never "it's over with us".
That said, don't feel bad, prepositions are the longest battle you'll have to fight with most languages you learn, and the fact that german has like 15 of them doesn't help. Next time, try learning French, they only have like 5 of them!
Odd, I find far more google hits for "It's over between us" than " It's over with us". Perhaps it's a matter of quoting the phrase. (With no quotes, I found the "with" results greater than the "between", but the numbers were far greater. I suppose search settings also have an impact.)
If I type 'It is over!' then is it really incorrect? I typed so, and it's been marked as a wrong answer. I'm asking, 'cos I'm not really sure. I interpreted 'Es ist Schluss mit uns!' as 'It is over!', for it is more common to say 'it's over' than to say 'it's over with us', isn't it? When we say so in everyday life, then don't we know what context is it used in, saying just 'It is over!'? I think we do, so...
And what I noticed since being a user of Duolinguo, is that the translation of sentences is in most cases very literal, intended to be similar structurally to German ones, no matter that it is not compatible with a real use of a sentence. I find that rather not good for a learning process, in my humble opinion, and 'It is over!' should be accepted as well.