Use one matching template
Choose the question count and number of options before printing. Keep those settings for the scanning run. The printed form contains four square corner marks and a separate top orientation mark; all five are needed to establish page coordinates.
Fill and capture clearly
Students should fill one bubble fully, avoid ticks or faint circles, and keep marks away from neighboring bubbles. Photograph the page upright and flat in even light. Include margins around every corner rather than cropping to the answer columns.
The reader is designed for this form, not arbitrary answer layouts. A sharp image of the wrong template is still incompatible.
Run a rehearsal before a class batch
Create a short sheet with 10 questions and 4 choices, print one copy at actual size, and fill a known sequence such as A,B,C,D repeated. Photograph that sheet and choose Read filled sheet photos with the same settings. Enter the known key and verify that the recognized answers match what you marked. This rehearsal checks the actual printer, paper, lighting and camera combination you plan to use.
The top of the form includes a small encoded question-and-choice marker in addition to its orientation mark. If settings differ from the printed form, the reader rejects the image. Do not crop, cover, redraw or decorate the top marks; their apparent visual simplicity does not mean they are optional.
Improve the capture before changing answers
Place the page on a flat surface, hold the camera reasonably parallel, and avoid a hand or phone shadow across the answers. The perspective transform can align a flat quadrilateral, but it cannot flatten a curled sheet. If corner detection fails, retake a sharp full-page image rather than manually entering every answer just to bypass a poor capture. Keep the original photo for review.