I like cognac [colour], but it involves a more thought to get right, than say a dark brown - such as Cleav is wearing in his images - which is a lot more versatile
. I'm not keen on double monks at all, although I would admit that cognac monks are a handsome shoe.
In terms of the sartorial rules / guidelines, since this is a HS graduation then you have a lot more leniency than a work environment (especially CBD). Your shoes will work with your suit, although a dark brown would work better and would be more versatile. The latter is important when you first start out and lack a basic wardrobe. By focusing on items that you can wear with a multitude of other items, allows one to get much more mileage out of a limited budget and wardrobe.
I would suggest another blucher rather than double monks straight off the bat, but given your age and that you are most likely not going to immediately enter the workplace, you can utilise the double monks with a smart casual outfit (navy blazer, grey or tan trousers / chinos, etc), as well as a suit - although there are better choices dependent on the level of formality required.
One suggestion would be to swap out the white shirt (very formal) for a light blue shirt. The white shirt intonates a level of formality that requires oxfords rather than double monks. A light blue shirt works with your suit and shoes, and is more relaxed than a white shirt. In any case, you will definitely need to wear a belt that matches
your shoes in colour.
As for ties, I believe that Cleav's gentle warning was to choose a sedate tie that complements both the suit and shoe colour, without being brash or gaudy. Feel free to peruse the Tie Basics 101
thread for ideas.
If you like how you look at your HS graduation and receive the compliments that your outfit deserves (you'll certainly be better dressed than most of your contemporaries - especially given that your suit doesn't look like you raided your dad's wardrobe and that your trousers are not puddling around your feet), feel free to peruse the SF 101 Primer
to get some grounding in the basics as you continue with your sartorial education.
Only other advice, is to be very careful when using the mensroom to avoid splash damage on your lovely new shoes.
If anyone thinks it would be funny to christen your new shoes, a pre-emptive slap works wonders.