True, economics was my worst subject in school, it never felt exact enough. I don't think they cover fractional reserve banking at the high school level, but it would be beneficial for everyone to understand it.
In the $ example, the depositor thinks they have $300 in their account and the borrower thinks they have $300 in their loan account, and they are both sort of right. Both can actually spend that money and transfer it to third parties at other financial institutions.
With bitcoin, each BTC is cryptographically signed. The bank cannot transfer the depositor's 300 BTC to the borrowers loan account without it disappearing from the depositor's account. Each BTC is unique, unlike $s which are essentially pooled by the bank to amortise the 'runs' on particular accounts. Am I misunderstanding? Perhaps banks could pool bitcoin in the same way, and the coins you transfer to your account are not necessarily the coins you use when you spend them, however this would seriously break the design of bitcoin.
All this shows why Bitcoin is not designed to replace national currencies. It is intended as a mechanism to transfer existing currency , although it behaves like a currency in various ways.