Another Scary Story

Students enter the biology classroom on Halloween and are greeted not by their normal teacher
but by Ms. Ivana Breakabeaker. At each of their seats, students find a strange message:
        Dear students,
        Your teacher needs your help! She was last seen at school yesterday but is
    nowhere to be found today. Authorities think she was kidnapped! The evidence
    left at the scene consists of one big blob of... uh, we're not sure what it is. Since it
    was here overnight, authorities believe that it has changed color. The four suspects
    authorities are considering for the kidnapping of your teacher are a witch, a ghost,
    a werewolf, and a vampire. Authorities have specimens on file (they have been
    suspects before!) in the following form from the suspects: wizard blood, ghost
    slime, werewolf slobber, and vampire tears. Your job is to test each substance and
    determine whether it contains sugar, starch, protein, or lipid. You will also test the
    substance left at the scene of the crime to compare to the suspects' samples. Then
    place the results on the kidnap note to see hidden text.

    It will tell you who must have committed the crime. Give this to the authorities to
    identify and incarcerate the perpetrator. Your substitute teacher, Ms. Ivana
    Breakabeaker, will help you today.
        Sincerely, Your Teacher, Mrs. Hokum

To help save their beloved teacher, students follow the procedure outlined below.

They must apply mixtures to the kidnap notes.

1) A witch blood containing oil (lipid)
2) Ghost slime contains gelatin (protein)
3) Werewolf slobber contains potato flakes (starch)
4). Vampire tears contain sucrose (sugar)

The blob is a combination of all four substances, so it should test positive for all four.
The blob is placed on the kidnap note. Text begins to appear.

It says,
Come to the town cemetery at midnight. We will release your teacher. At midnight the students
sneak out of their bedrooms and hurry to the cemetery. There, sitting dazed but okay is their
teacher, Mrs. Hokum. Unfortunately, the kidnappers are never found.