Is there a good substitute for the word "scarper" in American English?

I don't know how American it is, but "vamoose" has a few hits in COCA.

Perhaps absquatulate is the word you are looking for, although that might be getting on a bit now. You might also try skedaddle, which appears to be aging rather better.

'Scram!' or the old Bugs Bunny, Pig Latin version, 'Am-scray!'

I think book comes closest, both in meaning and degree of colloquialness, as in: "Look, John's coming in. I owe him money, so I gotta book. See you later."

Cheese it! The police!

or

I saw them coming up the street so I broke.

Maybe it's regional and obscure, but in that context I might use fly.

Quick, let's fly before the boss comes back.

Dang, I'm late. Gotta fly.

Let's beat it, just beat it...

Several options:

Let's get out of here before the boss comes back

Simple, easy to understand, and if you want to make it more informal, you can just add an intensifier such as "the hell" or some other flavor of the same.

Let's dodge before the boss comes back

Or

Let's get the .... out of Dodge before the boss comes back

I would have thought "scarper" was a reasonably well known word.

Apparently the original meaning derives from cockney rhyming slang: To "Scarpa flow" meaning to "go".

How about some of these:

"Leg it" "Make yourself scarce" "Vamoose" "Get out of here" "Blow this joint"

Or if your audience has seen Snatch (http://www.imdb.com/title/tt0208092/) you might like to say:

"Avi, pull your socks up!"

I hear "split" a lot but it's more correct to use when you are parting company. It wouldn't be as common when the entire group is leaving to move elsewhere (as a group).

Let's split before the cops find us.

I also hear "jet" particularly when time is pressing.

I hate to interrupt, but I gotta jet...
We gotta jet if we want to make the 10:15 show...

Scamper? Flee? ... Could be synonyms to "Scarper" in general...

Pretty sure the American for scarper is skedaddle. Amscray, the Pig Latin for scram, is also particularly idiomatically appropriate to your particular case.

Although personally a fan of "skedaddle," another common phrase for this is "bug out."

I can't believe no one's tossed "run" or "run for it" out yet. That's the word we used in my high school, not even a full year ago, when we joked about all running out of the classroom when the teacher stepped out for a minute, which I think is something like what you're talking talking about. :)

"Quick! (Let's )run for it before she gets back!" or just "Quick! Run! Before she gets back!" is what I'd say.

.....Although I have to admit "Lock the door!" was more common. ;)

I don't know how popular these are, but they are all American words meaning exactly what you're talking about (if they're not known it's probably because they're mostly used by high school kids running from the cops).

Dip (ie: Let's dip from the cops! They dipped out from the corner store.), Bounce (same thing), and Roll (more casual)

Personally, I've never heard "scarper". However, in (especially) American English, "scamper" is quite common, and would be the word I would use.

What is the origin of "scarper"?

Show your American Chums Harry Potter and the Prisoner of Askaban and ask them to check out Ron Weasley's rat Scarpers's behavior (running away). That would better aid them better to understand what Scarper means.

We'd use "jet", "ditch", "run", or "bounce" really depending on the day. In the context of a boss, I'd probably go with "jet" or "ditch".

"Let's ditch the meeting before the boss shows up..."

Dipset!

Let's dipset, it's the fuzz!

Beat it and scram are my top picks from the answers above. Here are my two cents:

Make a run for it

Let's blow before the cops get here. (as in blow like the wind)

Make like the wind

Let's get outta here.

Skip town before the cops get here.

Let's hustle before they find us. (very 70s)

I remember this from old British mysteries, I think either Dorothy L. Sayers or the Albert Campion books. Scarper is linked to gypsy talk (Romany). The gypsies said "scarpa." I would argue against "scamper" as the US equivalent. Squirrels scamper. Thugs scarper.