All teens wish to conqueror the world. The handwriting looks nothing alike.
No one argues that the handwriting is alike. The overall style is different, even if some letter similarities might exist.
Handwriting can be an affectation. Notes are often written to disguise true identity if someone wishes to hide it. Yearbook handwriting can also be an affectation, for reasons of putting on a show for posterity.
The "Shall Be The Conqueror" initials of SBTC is more difficult to explain away, unless it is one of those amazing coincidences in which the odds against it are one in millions or billions, but it happens anyway.
That high schoolers want to conquer the world, or that the letter I is left out, does not address the utter strangeness of these exact letters being the same exact letter placed at the end of the ransom note.
You and I didn't write "Shall Be The Conqueror" in our high school yearbooks and then many years later confess to a crime that had the initials SBTC featured at the bottom of a ransom note.
You knock down straw men. Knocking down a straw man argument gets nowhere when dealing with the astonishing appearance of these letters in these two situations.
I don't yet know what to make of this whole situation. I am still open to possibilities.