Why didn’t Marshall yell?

April's supposed to be babysitting Marshall while doing her homework. But before she can begin, she realizes that she's forgotten her math book; she left it back in Egypt, so she has no choice but to go get it. She wants to go by herself, but Marshall's having none of it: he wants to come with her. Reluctantly, April allows Marshall to tag along on her mini-excursion to the Professor's storage yard.
As April and Marshall are about to enter the yard, April's suddenly grabbed from behind by a big man. As we can imagine, April's absolutely terrified, but she can't scream because the man has his hand over her mouth. As for Marshall, he can't scream either. As he tells April later on, he couldn't yell because his throat was stuck. Thank goodness the Professor was watching all this happening and was able to call for help. Otherwise, April and Marshall would've been in real danger that night.