Severus Snape wasn't the bravest wizard in the world. Forty years from now in another time-line, certain Harry Potter would have said to his son that him, Severus Snape was probably the bravest man he had ever known. Alas this would never come to pass now. Severus Snape was intelligent, smart and some, who were able to look past their prejudice, would have even called him brilliant. But he wasn't brave. Fortunately, who you are, is not necessary who you may become. In some individuals bravery could be nurtured and taught. However unfortunately for him this was now and after adrenalin was out of his system he was scared out of his mind. This was the reason why he didn't notice the fear in the eyes of several professors present in the office of Headmaster, this was why he didn't notice a deeply disturbed and worried expression on the normally unfazed face of Hogwarts Headmaster.
If only he had bothered to invent the counter-curse, he could have set everything right, Severus wished for the tenth time that day. Dumbledore, being a skilled legilimens didn't even need to probe the head of the boy in order to pick up that particular thought and that was the main reason why the frown on his face dominated over carefully concealed anger. At first he had planned to simply expel the boy and be done with it, after he found out precisely what curse was inflicted on James Potter, of course. Madam Pomphrey still needed to properly heal poor Gryffindor. But this little piece of information changed everything.
"Was the spell of your own invention, Mr. Snape?" – he carefully tried to direct conversation to the subject of counter curse, without being too obvious.
Severus Snape, surprised that headmaster already knew about it and still didn't expel him, composed himself a bit and laconically answered with a shaky voice:
"Yes, sir."
Fear still had a firm grip over him so he completely failed to notice how Professors McGonagall and Slughorn exchanged a meaningful look.
"I trust you have also invented the counter curse, boy?" – Dumbledore pretended his didn't know the answer already and was even a little amused by the irony of it all, when Severus lowered his head and shoulders in defeat, preparing for worse:
"No, sir."
Dumbledore looked with a grave expression upon his student who didn't even dare to face his eyes, and wanted to ask him:
'Do you realize what you have just done, young man? Do you realize that without a counter-curse Sectumsempra is even worse than unforgivable, since it doesn't even require a bloody intent? Do you realize that it would take me years to invent counter-curse myself? Years, that we don't have. Do you realize that if I expel you from Hogwarts and leave you to your own devices, you will be in Voldermort's clutches faster than you can say 'Dark Mark'?' – instead he has said this:
"Mr. Snape, I will grant you a second chance and allow you to redeem yourself by cleaning the mess you have created. You will be suspended for the maximum period of two weeks." – Severus lifted his head and looked at Headmaster with utter shock and disbelief. Dumbledore realized next part will be crucial and summoned as much intimidation into his voice as he could master:
"If you fail to invent the counter curse by the end of this period you will be expelled from Hogwarts" – he lied, in fact he was going to expel him if he succeeded and handle him over to Alastor Moody for subsequent indoctrination and drilling in Auror's academy. Order of the Phoenix could use some talent in the upcoming war. – "Your head of the house will personally oversee your suspension and you will be isolated from the rest of student body. Now get to work ." – Dumbledore finally dismissed him.
"Thank you, sir" – Severus replied with confused expression on his face, finally able to think for the first time, since the fear has passed. What the hell has just happened? How did he manage to avoid his just punishment? He would spend countless nights thinking about it, but he was still too young to realize that maybe in the perfect world where everyone would have the same set of values and beliefs, maybe, just maybe everyone would get what they deserved, but in the world with Voldemort on the loose, trying to impose his own set of values and beliefs, good things sometimes happened to brilliant wizards, willing to spill some blood.