Final answer:
The teacher should add two conditions in the conditional formatting: one for 'greater than 90' and another for 'less than 100' to highlight cells with values in that range.
Step-by-step explanation:
If the teacher wants to highlight cells with values greater than 90 but less than 100 in a spreadsheet, the correct step to take after selecting 'Format' > 'Conditional Formatting' would be to click on "greater than" and enter 90.
Then, the teacher should apply a second condition by selecting 'Add' again, click on "less than" and enter 100. It is important to ensure both conditions are set in the conditional formatting rules to accurately highlight the cells that meet the criteria of being more than 90 and less than 100.