How can I quickly return to Singapore after being denied entry due to testing positive for THC at the airport?

Not a lawyer, not your lawyer, just a random person who has lived in Singapore for a long time.

Singapore's drug policies are famously strict; however, the reason you have escaped severe penalties is because it's not entirely clear you broke any drug laws.

• Consuming drugs outside Singapore is an offense if and only if you are Singaporean or permanent resident (Sec. 8A), which you are not. So failing a drug test is not, in itself, an offence if the drugs were consumed outside Singapore. (Compare with this guy, who tested positive while working in Singapore.)
• Singapore bans all derivatives of Cannabis sativa including THC. However, CBD in itself is considered a (non-permitted) health supplement, not a controlled substance, and there's even a government-sponsored program for research on synthetic CBD.
• It's an offense to possess any "pipe, syringe, utensil, apparatus or other article intended for the smoking, administration or consumption of a controlled drug". Does your grinder qualify? Maybe, maybe not, depending on whether they can link to intent to consume controlled drugs.

It thus appears that instead of incurring the expense and hassle of taking you to court with a less than watertight case, they opted to simply deny you entry. So the good news is that it's unlikely that there are any drug charges waiting for you in Singapore, since if there were, they would not have let you go.

The bad news is that since you were refused entry, you are now almost certainly blacklisted from Singapore. The period of ban is not disclosed, but it's typically at least 6 months and can be up to 7 years, meaning that you can almost certainly forget about joining your campus program in the next four months.

If you ever wish to return to Singapore, even in transit, I would strongly recommend engaging an immigration lawyer and applying for a visa instead of simply rocking up and hoping for the best.

Like everyone said in the comments, don't ever try to return to Singapore, at least not without getting a specialized lawyer in Singapore's law (the embassy may have a list of such lawyers).

If you don't have the proper authorization/rehabilitation, you will encounter a whole lot of trouble (according to this site, you may risk 1-3years in jail (likely on top of your existing offense) and up to S$6000 (~4000€)).

The fact you committed drug offense and they deported you, may bar you, from entering some countries (

• US : Commiting a drug violation renders you inelligible under INA section 212(a)(2)(A)(i)(II) for life (source)

• Canada : Renders illegible someone that has criminal or immigration-related offenses for at least 5 years and until they get the appropriate waiver (source)

• UK : The UK doesn't outright ban criminals from getting a Visa, but the officer will look very suspiciously at the application

It goes with sense that you aren't eligible for a single visa waiver program anymore

...)

Again, it can depends widely on what exact grounds they deported you... But the simple fact of being deported will make you have a lot of trouble to get visas...