Malicious Intent

We're sitting around the wall of the gym, our sport class just having finished. We're all waiting for the bell to go, Isabelle talking to me about the game we just played.
“It's ridiculous!” she's saying, brushing her blonde hair out of her face. “They always hog the ball. It's like we're invisible or something.”
“That is a bit irking,” I reply, checking the time on my watch.
“Anyway, what do we have next?”
“Break, then science.”
“Thanks.”
Something on the oval catches my attention, and my deep green eyes focus on it. I could have sworn I saw something move. Maybe it was a bird, but there's not a single one in sight. There isn't any birdsong either, the area silent aside from out class' chatter. Weird... For some reason it just feels wrong.
“What are you looking at, Steph?” Isabelle asks, leaning over to see from my perspective.
“Nothing. That's the problem,” I murmur, narrowing my eyes.
She frowns. “I don't get it...”
“Do you hear that?”
She listens for a bit, then shakes her head. “Hear what?”
“That's just the thing. There should be birds or insects or something, but there isn't.”
“Steph, I don't know what you're trying to say,” she says airily, but I can see she's as confused as I am now.
I'm about to say something else when the clear space I'm staring at on the oval shimmers. Like, actually shimmers. The air glints a weird rainbow colour with a deep green hue to it. I blink, hoping I didn't actually see that. Then I realize that the class has gone quiet, mumbling about what just happened. So it's not just me going mad, then.
It does it again, longer this time, and I can see that it's like a rectangular panel, hovering half a meter off the ground, about four feet wide and eight feet high. It shimmers again, pulses, then stays. It's like a window of green light, flecks of different colours running through it in little patches.
Nothing happens for a moment. The light is still there, just hanging in the air, and people are staring at in awe and disbelief, murmuring about it, asking what they think it could be.
Our sport teacher, Mr.Grennan, tells us to stay put, then slowly walk over to it. He stops about a meter and a half away from it, examining it, then walks around it in a big circle, stopping when he's in front of it again.
That’s when things get really bad.
Something steps out of the window of light, and everybody freezes, going perfectly silent. The thing is about seven feet tall, grotesque and covered in thick hair. Its body looks like it’s made from parts of multiple animals –I can see that its head is like a deer, ape-like arms, a scorpion’s tail –and is covered in a slimy substance I’d rather not think about.
It screeches, though it has no visible mouth. It sounds like a triumphant, deformed howl, agonizing to the ears as it rings out across the oval. Its blank white eyes, having no pupils, settle on Mr.Grennan, and it squeals. Its tail lashes out and before he even realizes it the stinger is buried deep in his head. It flicks the tail back, bits of his skull and brain littering the grass.
Silence follows. Nobody moves, barely even breathes. The bell rings, and the beast screeches, stretching its claws and glaring at us.
Then the whole class screams and runs for their lives.