Okay so maybe I'm getting too excited about world building but we do need a cohesive magic system, so...

Spells
Spells are cast orally, and there is a few different ways to do it.

WitchSpeak is the oldest method, and it's an ancient language, though most witches understand it intuitively after a little study.

Spells can also be cast in English(or any language) though they tend to be less powerful this way, and there's a higher chance of the spell backfiring.

Runes

Runes are magical symbols drawn or carved into an object. They serve to cast long lasting enchantments or curses over places/objects. Most majors are required to at least take 'Intro to Runology," though it's an uncommon form of magic because any complex runes require study and practice. Additionally, it's hard to cast a spell using runes on the fly unless you're highly trained in writing them accurately and quickly. That said, if you're skilled at drawing in runes, it takes significantly less energy to use them when compared with a traditional spell.

Runes are found in several parts of campus, and most dorm rooms have runes carved above them for protection.

Magical Energy

Magical Energy is the innate ability of a witch to perform magic. It's not a tangible thing, but it is sometimes (not often) visible in a glowing residue that lingers in the air when spells are performed. 

This energy likes to be exercised, and many witches who suppress their ability will report a pain in their joints and hands, as well as a faint glowing underneath their fingernails or around the cracks of their teeth.

Energy is depleted with use, which is why excessive magic use is exhausting.  Casting certain spells, or spells you aren't familiar with, is more tiring than casting others. Like running or waking, though, overusing your energy is rare, as most sensible people will stop when they begin to feel fatigued.

Potions
Potions function exactly like you'd expect them to. Cauldron, weird ingredients, super specific directions... the works.

Technology

Magic and technology don't conflict. Most witches use cell phones and watch TV. Some even enchant their electrical devices.

Demons

Demons are not necessarily evil. In fact, most witches use the word as an umbrella term to encompass all supernatural beings that possess a consciousness. Technically, familiars could be considered demons, as could angels.

Devils, however, are a subset of demons, and the term refers to those evil creatures which serve Satan (The Devil) and try to convince you to sell your soul. Devils can briefly take on a humanoid appearance, though they have to possess someone to remain on Earth for very long. Most "demon lore" applies to devils. They don't like salt and iron, and they tend to hang around crossroads. Devils are pretty dangerous for witches who aren't specially trained in dealing with them.

Before anyone asks, no, a devil (or any demon) cannot reproduce with a witch or human.