As already mentioned, "tank" is a bit of a tricky term to define in the context of Istaria and how the agro works. Normally, a tank is there to soak up damage, in which case, any class which gets 10 armor use per level and can wear Platemail would typically fit this description. Cleric being the most common suggestion thus far due to their healing capabilities, BUT they don't get 10 armor use per level, so you'd still have to level another school to be able to wear the T5 platemail.
So, if you were looking for your first class and wanted to be a tank, then Warrior would be the only one that gets 10 armor use per level and can wear platemail.
If you're trying to plan your schools and want a diverse "tank", then I'd imagine most people would suggest Cleric. Schools which would support Cleric are Warrior (armor use, strength, weapon skills, some masterables), Druid and Spiritist (for some excellent magic synergies i.e. AoE and life steal), and Healer will boost your healing capabilities (Life skill and Focus increases). This is by no means a definitive list, and it may not work well for your play-style.
I've always been thoroughly impressed with what a high rating Cleric can manage, but for me, Druid has ALWAYS been my go-to
It's a hell of a trade-off between a multi-classed Cleric to Druid, with considerably less armor just for Cloak of Thorns. Paladin can certainly work well, but they only have access to the two revitalize spells, so you take a significant hit to your healing capabilities and the increase in damage is not always a suitable trade-off.
Anyway, just some more food for thought.
Just as long as you enjoy the school, then you can't really go wrong