Checking on Raspbian 8 (jessie):
> apt-cache search mysql-server
mysql-server - MySQL database server (metapackage depending on the latest version)
mysql-server-5.1 - MySQL database server binaries and system database setup
mysql-server-5.5 - MySQL database server binaries and system database setup
mysql-server-core-5.1 - MySQL database server binaries
mysql-server-core-5.5 - MySQL database server binaries
This is the same as wheezy (actually that's an upgraded to jessie system; if you have a new from scratch jessie 5.1 will not be there). So in fact 5.6 is not available. Note that Raspbian binaries are compiled for the ARMv6 platform -- you cannot use just any Debian package. The current Raspbian (jessie) is parallel to the current Debian anyway.
5.6 is available for "stretch", the current Debian testing branch, and as Peter Green notes in a comment, it has at least been built for Raspbian.
Along the lines of the methodology described here, you can try and install a stretch package on a jessie system. I.e., add:
deb http://archive.raspbian.org/raspbian/ stretch main
/etc/apt/sources.list. Then run
apt-get update. Finally:
apt-get install mysql-server-5.6
This requires various stretch dependencies and a few upgrades but is worth trying.
It also is in theory possible to upgrade the whole system now (from jessie) if you change
apt-get update and then (this is the edge of the cliff...)
apt-get dist-upgrade (which will probably take some time), but beware this has probably not been tested much by the masses yet.
If you have a Pi 2, 5.6 is available for pure ARMv7 distros -- primarily Ubuntu (it is probably an alternative in 14.04 LTS). The default on Fedora 21 ARM is 5.6.26.