Not all group two hydroxides are considered strong bases.  Just like not all the hydrogen halides are considered strong acids.  However, barium and strontium hydroxide are indeed very strong bases.  It's just a matter of how far down in the group they are.  Also, just because something doesn't dissolve in water doesn't mean it's not a strong base.  For calcium hydroxide, the portion that dissolves in water pretty much is 100% dissociated.  It would be considered a weak base if it did dissolve in water but didn't completely dissociate.  Take a look at ammonia.  Ammonia does dissolve quite readily in water, but it doesn't fully react to completely form OH- ions.  You wind with a lot of NH3 in the water.  So the determination of the strength of the base can also depend on how much of it dissociates when it does dissolve in water.  Just because it has a low solubility doesn't make it a weak base.  (Overall, Ca(OH)₂ is kind of a mid-level base in terms of strength).

Not all acids dissociate completely. Weak acids don't and yet there still dissolved in solvent.

Thanks! But how can something dissolve without dissociating?

Well, take a look at sugar.  That dissolves in water but it remains in water as the sugar molecule, not something else.  Or look at acetic acid.  Acetic acid will dissolve in water, but it a good portion of it remains in the water as the pure acid, and only a small bit dissociates into H+ and the acetate ion.