I will take a shot at helping explain this to you guys, at least this is how I understand it. When you take out a student loan, you can either pay the interest charges while you are in school or you can have them added to your loan. If you choose to pay your interest charges each month, at the end of college you will only owe what you have borrowed. However, if you choose not to pay them, each month your interest gets added to the principal amount you borrowed and the principal of the loan creeps upward each month. Since these interest charges are added to the principal, you have more principal accruing interest each month so your interest charge increases slightly each month. The interest does not sit in some separate pile that does not accrue interest.
As to your specific case, if you pay your interest each month, your principal will only increase when you take out additional loans. Therefore, assuming you take loans out each semester, your interest payment will only go up twice a year, when you take out your semester loans.
It sounds like you want make the same payment amount each month, which complicates things slightly. To do this, you will want to calculate your average debt load for vet school. If you start with maybe $10,000 in undergrad debt and expect to finish with $90,000 in debt, then your average debt load through vet school would be roughly 10,000+90,000/2= $50,000. If your interest rate is, say, 6%, then your average interest charge would be $50,000 x.06= $3000 a year. Divide that by 12 months and that is the amount you would want to pay every month.
If you paid that every month during vet school, your first two years a portion of that payment would go toward the principal, since your interest charge wouldn't be as high as your payment yet, and the second two years a portion of your interest charge would go into the principal, since you are paying less then the interest charge. But these two factors would roughly balance out so that when you are done you would only owe roughly what you borrowed.
This would get you close to what you need to pay per month to cover interest. Unfortunately, if you plan on owing more than $100,000 when you graduate, $200 a month probably wont quite be enough, but it is a lot better than many people do. Keep in mind also that, I believe, the subsidized stafford loans we got up until now do not accure interest until you graduate, so that will help. Also, I am not a CPA and don't even like balancing my checkbook so don't hold me to any of this, but it is the way I understand the system to work.
