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 did n'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 drill field near the newly christened memorial , Aaron Friedman said the summer break was a welcome reprieve , but he was ready to return to Blacksburg . `` It was good to get away . Being back with everybody at home really helped , '' Friedman said . Fellow sophomore Mike Giancola said he , too , is ready to move on , even if he is still disturbed by memories of the shootings . `` I still think about it every day , but I 'm ready to get my life going . '' Sheehan echoes the sentiment of Giovanni and her fellow students but noted that she made some changes in her life , and she still battles post-traumatic stress symptoms every day . `` Even being in my new dorm , when I hear a loud noise or something unexpected , I 'll peer out through the little hole in my door to make sure everything is OK , '' she said . E-mail to a friend . CNN 's Reggie Aqui , Brianna Keilar , and Gary Nurenberg contributed to this report .
Student says she avoided being shot during massacre by playing dead . No classes will be held in Norris Hall , where 29 of the victims were killed . Professor : ` ` I think that we 're very excited that classes are starting '' and then they start shooting at us from inside''.