They say that since a friend who goes to college uses a desktop.
I am shaky on the whole desktop idea though due to the portability.
FWIW, I teach at a local community college as an Adjunct Faculty member (i.e., part time, general-level courses). So I am NOT talking about fields involving advanced modeling, nuclear imaging, statistical analysis, computer-aided design, cryptography, fluid dynamics, etc. - instead, I'm talking about
basic general education program requirements.
Instructors at even 100 level classes will often expect you to have computer proficiency skills, and access to a computer. This expected access sometimes requires mobility. Much of the work you will do in college will be in a team (especially in a 4 year university) - in many instances, you will be doing work over someone else's dorm/house/apartment/etc., and you may find yourself frequently at a campus hotspot where people meet to work together and study. A desktop will not work here. You may be able to rent a loaner laptop, but this is not very convenient (and may not be very secure). Ultimately, a desktop = limited mobility. Without a laptop, you may very well be at a disadvantage compared to those that have one. A laptop, especially a Mac, is an educational tool for success. Compared to the pricing of tuition/housing/food/travel/etc., it's a pretty small investment in the grand scheme of things, but one that can make a big difference in academic achievement!
Further, many teachers now speak/move at a pace that follows typing speed, and NOT writing speed. They may move faster than you can write. While you could record lectures, being able to type it on the spot is more time efficient.
Ultimately, if your parents are worried about financials, that $650 NIB MacBook Air is hard to beat - add on a 3-year accidental damage plan for 125-200 bucks and you are protected against hardware failure, drops, AND liquid damage! If they are worried about power, they should not worry about this because you likely have several years before you will need such power processing for school-related activities...and, when you do, you will have access to workstations (or even supercomputers, depending upon your field.) Power is really unimportant compared to mobility and reliability at this point IMO, and is a major reason I suggested a MacBook Air as a lower-cost option because it is extremely reliable, has great battery life, and is very mobile.
I cannot stress to your parents and yourself enough how pivotal a laptop is for success. Macs are great for this role (and, for that matter, so are many Chromebooks and Windows PCs, but IMO the Mac is better for this purpose.) The most important thing is realizing this tool has moved from a luxury to a necessity, and often many instructors are assuming you will have the mobility with this essential tool that only a laptop or tablet hybrid can provide.