The problem is that the cin ">>" operator parses text delimited by whitespace. So it reads in Tony, then sees the space, stops parsing. " Stark 90.25" will still be in the buffer, so it will attempt to parse "Stark" into your studentGrades element.
One option is to use getline, because getline is delimited by the newline instead of spaces. But this would be easier to use if the 90.25 part was on its own line.
If your format is always (First Last Grade), then perhaps it's easier just to use three variables.