Shakespeare is easy in comparison to say Chaucer, or other 'olde' English texts. If children leaving secondary school could read Shakespeare - obviously one of the easier plays - we'd see a dramatic increase in the quality of written and spoken English.
I feel strongly about this as a Literature graduate and will concede that I look at this from a biased viewpoint, but at GCSE I never considered Macbeth for example particularly hard-reading.
So it is almost certainly the teaching we should be examining.
There is no way Macbeth is remotely easy-reading to the average kid. I mean, once you look up all the words it's OK, but lets take this extract from Act I as an example.
Doubtful it stood;
As two spent swimmers, that do cling together
And choke their art. The merciless Macdonwald--
Worthy to be a rebel, for to that
The multiplying villanies of nature
Do swarm upon him--from the western isles
Of kerns and gallowglasses is supplied;
And fortune, on his damned quarrel smiling,
Show'd like a rebel's whore: but all's too weak:
For brave Macbeth--well he deserves that name--
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,
Like valour's minion carved out his passage
Till he faced the slave;
Which ne'er shook hands, nor bade farewell to him,
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements.
Give that to a 15 year old and tell him to read it and see how much he genuinely understands. It's a doddle compared to Chaucer, but going through a whole play of that is difficult stuff. And Macbeth is by no means one of his harder plays to get through.