Your teacher may be using the tactic of making you argue against your own position in order to help you spot potential weaknesses in your own position and thus ultimately strengthen your own argument. In any case, here are some quotations from the play that may imply that Macbeth is already politically ambitious even without the witches' influence:
Macbeth. Your children shall be kings.
Macbeth. And thane of Cawdor too: went it not so?
Macbeth. [Aside] Glamis, and thane of Cawdor!
The greatest is behind.
Macbeth. [Aside]. Two truths are told, 240
As happy prologues to the swelling act
Of the imperial theme.—I thank you, gentlemen.
[Aside] This supernatural soliciting]
Cannot be ill, cannot be good: if ill,
Why hath it given me earnest of success, 245
Commencing in a truth? I am thane of Cawdor:
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature? Present fears 250
Are less than horrible imaginings:
My thought, whose murder yet is but fantastical,
Shakes so my single state of man that function
Is smother'd in surmise, and nothing is
But what is not.
Macbeth. [Aside] If chance will have me king, why, chance may crown me,
Without my stir.
Banquo. New horrors come upon him,
Like our strange garments, cleave not to their mould 260
But with the aid of use.
Macbeth. [Aside] Come what come may,
Time and the hour runs through the roughest day.
Macbeth. [Aside] The Prince of Cumberland! that is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be, 335
Which the eye fears, when it is done, to see.
[Enter LADY MACBETH, reading a letter]
Lady Macbeth. 'They met me in the day of success: and I have 345
learned by the perfectest report, they have more in
them than mortal knowledge. When I burned in desire
to question them further, they made themselves air,
into which they vanished. Whiles I stood rapt in
the wonder of it, came missives from the king, who 350
all-hailed me 'Thane of Cawdor;' by which title,
before, these weird sisters saluted me, and referred
me to the coming on of time, with 'Hail, king that
shalt be!' This have I thought good to deliver
thee, my dearest partner of greatness, that thou 355
mightst not lose the dues of rejoicing, by being
ignorant of what greatness is promised thee. Lay it
to thy heart, and farewell.'