BLACKSBURG, Virginia (CNN) -- Erin Sheehan is one of the almost 28,000 students making the bittersweet return to the Virginia Tech campus for fall classes, a journey she was afforded only because of some quick thinking in April.



Students return to the Virginia Tech campus Monday for the first day of fall classes.



When Seung-Hui Cho stormed into Sheehan's German class that horrifying Monday morning -- firing off indiscriminate rounds of gunfire that killed the German teacher and four of Sheehan's classmates -- Sheehan hit the deck and played dead.



"He went around the room shooting everyone," she said.



Cho's April 16 rampage through Norris Hall left 29 students and faculty dead. The mentally disturbed Cho also killed two others and himself in the mass shooting, the worst in modern U.S. history.



Though she is heading back to class, Sheehan is admittedly still shaken. Four months after the tragedy, Sheehan is haunted by memories of the massacre, and she still jumps when she hears a loud noise in her Virginia Tech dorm room. Regardless, she is prepared to move on.



"When classes started again in the spring I didn't come back," Sheehan said. "It was just too hard to do then, but now I'm ready."



The campus was buzzing over the weekend. Parents and students crowded the sidewalks, toting boxes and furniture into the dorms, including West Ambler Johnston, where the first two victims were killed.



Norris Hall, meanwhile, has been refurbished and will no longer host classes.



Virginia Tech has discreetly beefed up security since the tragedy. Locks that operate from the inside have been placed on classroom doors. Dormitory entrances are secured 24 hours a day. Students can sign up for emergency text messages from the university.



Despite the precautions, not everything has gone as smoothly as planned.



On the eve of the students' return to classes, 23 people fell ill from a carbon monoxide leak in an apartment building near campus. Five Virginia Tech students were hospitalized, police and hospital officials said.



Blacksburg Police said a valve on a water heater that was malfunctioning is believed to be the cause of the incident. Police said the valve was stuck in the open position, causing a constant burn-off of fuel and creating carbon monoxide that was not ventilating.



Kirsten Halik and Kristin Julia, both 19, were upgraded from critical to serious condition Monday and Elizabeth Burgin, Carolyn Dorman, and Nichole Howarth, all 19 -- are conscious and alert, according to hospital officials.



Burgin, Dorman, and Howarth were upgraded from serious to good condition, hospital officials said, after they received treatment in a hyperbaric chamber on Sunday and Monday. The chamber pushes pressurized oxygen into the tissues and blood.



Meanwhile, the university dedicated a permanent memorial to the victims and their families -- a semicircle of 32 engraved stones bearing each victim's name.



One student said she wanted to attend the ceremony until she woke up at the Collegiate Suites apartment building feeling nauseous, The Washington Post reported.



Spirits are high, however, said professor Nikki Giovanni, and students and faculty are ready to move past the tragedy.  Watch Giovanni applaud the university's new security measures »



"The Hokies' spirit is alive and well. I think that we're very excited that classes are starting -- that we're moving into a new year," she said.



Perhaps indicative of that resilience is the size of the incoming freshman class. According to a university news release, the university accepted deposits from 5,215 freshmen this year.



That is more than its enrollment target of 5,000 and about 30 more deposits than the university received from incoming freshmen last year, the news release said.



"Virginia Tech took a big hit, but I think that we have embraced each other. We have been embraced on this planet," Giovanni said, explaining that the Virginia Tech community has received greetings "from every country on Earth and it's been a wonderful thing."



Students concur they are ready for the new school year.



Playing Frisbee with friends at a
What did the student do so as not to be shot? Where will no classes be held? What is the number of students sick from carbon monoxide? What type of leak left 23 sick? How many victims were killed? Who played dead to avoid being shot? In what way did the student avoid being shot? What is the number of dead?
Sheehan hit the deck and played dead. Norris Hall, 23 a carbon monoxide 29 Sheehan played dead. 29
question: What did the student do so as not to be shot?, answer: Sheehan hit the deck and played dead. | question: Where will no classes be held?, answer: Norris Hall, | question: What is the number of students sick from carbon monoxide?, answer: 23 | question: What type of leak left 23 sick?, answer: a carbon monoxide | question: How many victims were killed?, answer: 29 | question: Who played dead to avoid being shot?, answer: Sheehan | question: In what way did the student avoid being shot?, answer: played dead. | question: What is the number of dead?, answer: 29